The NHS has been encouraged to weed out patients who do not need to be on the waiting list, described as "validation" by officials, for a number of years.
Financial incentives are paid for the number of patients taken off and can also include patients who have died.
It can make services more efficient, as it means hospitals are not unnecessarily chasing up patients who no longer need treatment.
Internal documents showed that, during April and May, the waiting list was brought down by 100,000 through weeding out patients. Without this the list would have increased.
The Nuffield Trust think-tank said the true number being removed could be even higher.
Its analysis showed an average of more than 200,000 removals a month over the last two years. The think-tank claimed that, on top of validation, computing errors could mean some patients are being automatically removed in error, creating an additional waiting list that does not show up in the figures.
Dr Becks Fisher, from the Nuffield Trust, said: "It would be easy to assume that recent reductions in the waiting list are down to the NHS treating a bigger number of patients each month but our analysis shows that presenting progress in this way is a mirage.
"The NHS is actually still treating fewer patients than are being referred."
She said while this waiting list "validation" exercise was "absolutely" the right thing to do in many cases, there needed to be greater transparency and understanding about just what is happening behind the scenes.
The government said validation removals were only a small factor in the progress being made on the waiting list, pointing out the NHS was still managing to treat an increasing number of patients.
Screening the waiting list through the validation process was vital and ultimately improves productivity, they argue.
A Department of Health and Social Care spokesperson said: "Our drive to clear the huge waiting list backlog we inherited includes making sure all patients are getting the right treatment as quickly as possible."