Campaigners want an inquiry into how the the Independent Police Complaints Commission (IPCC) collates its figures.
The IPCC says its tight definitions allow it to track trends.
The findings were revealed in a joint investigation by BBC Radio 4's File on 4 and The Bureau of Investigative Journalism, which used Freedom of Information requests to ask the IPCC to reveal the names of individual cases detailed in its statistics.
The names relate to 86 people who died in police custody between 1998-9 and 2008-9 following the use of restraint.
Of these, 16 deaths were categorised by the IPCC as being directly "restraint-related".
'Skewing the results'
Families of those who have died have expressed disbelief that their loved ones have been excluded from death in custody figures.
What we will have to do is have a proper thorough inquiry into this matter”
Keith Vaz, chairman of the Commons Home Affairs Select Committee, said the findings were concerning.
"This is a highly sensitive area which deals with one of those parts of public policy that needs to be looked at very carefully," Mr Vaz told the BBC.
"What we will have to do is have a proper, thorough inquiry into this matter."
Mr Vaz said the IPCC had a crucial public role.
"It is the organisation that the police and the public turn to in order
to get a definitive account of what happened in respect of some of the most serious cases that there are."
Mr Bosworth, a property valuer from Peterborough, died after being restrained by the police in July 2008.
An inquest jury returned a narrative verdict.
Mr Bosworth suffered a fatal heart attack brought on by a combination of being restrained and his cocaine use and epilepsy.
Despite the fact the IPCC conducted its own investigation into Mr Bosworth's death, which cited restraint and struggling as part of the cause of death, his name is not included in its custody death figures.
Tom Bucke, head of analytical services at the IPCC, conceded that it was an important case.
But he defended the commission's position not to include the case in the figures because Mr Bosworth had not been formally arrested or detained.
The watchdog disclosed it has another list of deaths following police contact - those who had not been arrested or detained.
However the IPCC does not know how many of those were restraint-related deaths and is considering a new study of these cases.
The investigation carried out by Radio 4's File on 4 and the Bureau of Investigative Journalism into the official figures involved months spent cross-referencing the names on the IPCC list and other cases in the public domain.
The IPCC's definitive list of 16 deaths in police custody excludes one of this country's most high profile cases.
Roger Sylvester died in 1999 after being restrained by eight police officers in hospital.
He had been found naked and behaving strangely outside his home in Tottenham, north London.
An inquest ruled in 2003 that he was unlawfully killed, but this was quashed a year later by a High Court judge. No officers were charged.
But following the case, the Metropolitan Police reviewed and reorganised its restraint training.
'Led by evidence'
Deborah Coles, chief executive of Inquest, a charity which advises on contentious deaths and their investigation, said it was "absolutely astonishing" that Mr Sylvester's death was not on the list of 16.
I know these cases, we've worked on these cases, and restraint was absolutely fundamental during the course of that inquest” - Deborah Coles Inquest
"I find that absolutely astonishing because I know these cases, we've worked on these cases, and restraint was absolutely fundamental.
"I would question the IPCC as to how these figures were collated, and what care has gone into ensuring that they're properly representative of... the investigations, some of which they have been directly involved in."
The IPCC said Roger Sylvester's name had been excluded from the list again due to its tight definition of restraint-related death.
Tom Bucke of the IPCC said was restraint was a "key factor" in the case.
"However, we are led by the evidence and the medical evidence was disputed by a pathologist at the inquest and the final inquest verdict was an open verdict on Roger Sylvester's death, so under cause of death we reflect the open verdict and the dispute between the pathologist about that," he said.
"The reason we have such a tight definition for our different categories is to allow us to look at trends over time.
"If we had very loose categories and included lots of things in there which may or not fit in there or may fit in other categories, then we would lose the ability to say whether deaths were going up or down.
"It's obviously very important for everybody concerned to know whether deaths in custody are going up or not."