This is because the ATP count the TB where they have the scorelines, by this is than older players have some TB in their own page.
I think you'll find, if you check, that Sampras' Challenger TBs did not have scorelines. Still they are counted. But for Nastase, it is correct. He has 20 TBs with scorelines - and rather a lot of them without.
I suppose it would be too much to expect ATP to show the same diligence in counting ATP TBs as they do counting Challenger TBs.....