ACC has revised the number of claims it refuses to about 90,000 a year - 20,000 more than it previously claimed.
Despite the correction, researchers and advocates say ACC is still grossly underestimating the numbers of claimants unfairly missing out.
In May, a report by Dunedin lawyer and researcher Warren Forster concluded up to 300,000 people a year could be missing out on ACC cover, treatment or support because their claims were being declined.
At the time, ACC chief executive Scott Pickering disputed that figure.
"When I look at the numbers that we have, we accept about two million claims per annum and ... we would decline around 70,000 per annum," he told RNZ.
But Mr Pickering accepted that part of the reason for the difference was that ACC was not including cases where just part of a claim was rejected.
Since May, ACC and Mr Forster have tried to resolve the gulf between the figures.
ACC has now lifted its estimate of refusals to 90,000, which customer officer Mike Tully said took account of some of the problems raised by Mr Forster.
"We've gone through on the information we've got, we've looked at all of that information, we've done a review of those numbers as highlighted by Mr Forster if we included the extra areas that he was alluding to in some of the discussions.
"So we went and looked, and that's where that number moved from the 70,000 up to the 90,000."
Mr Forster said that did not go far enough, and ACC should hand over the data for proper scrutiny.
"There's a massive increase between 70,000 and 90,000, which is a 29 percent increase off the bat, but it's about time that actually they either front up and give us the data or accept that they don't have the data."
Mr Forster was adamant there were still tens of thousands of people missing out.
Denise Powell, from ACC claimant support group Acclaim Otago, said the changing figures did not give her a lot of confidence about the way ACC gathered data and what it kept.
"With a flip-flop of figures, it does make you think 'well, I don't know, which is the right figure' and why can't we get a definitive answer because we should be able to," she said.
Mr Tully said ACC stood by the new figure.
"When we've done the work, these are the numbers that ... are representative of our declines in these areas," he said.
ACC is updating its IT systems, and Mr Tully said that would make if easier to keep a more accurate record of claims.
But he said the new system could also reveal that more people were missing out on their entitlements.