The ranking system has changed many different times.
Prior to 1990, it was the average what was computed (i.e. the points divided by nš of tournaments played), that is why Connors was nš1 in 1977 ahead of Vilas (his average points per tournament was a bit higher than Vilas's). There was also the complication that there were some tournaments (invitationals and others) that had good draws but did not have points to the computation of the ATP rankings.
In the 90s, the best 14 results (in terms of points) was what was computed (and NOT the average like before, i.e. not divided by 14 or nš of tournaments played). A good fraction of the points any player had, were BONUS points. You gained points by advancing rounds AND beating ranked players. If a player won a tournament beating several top-10 players, especially, beating the nš1 player in the world, he could obtain as many points by BONUS as for the tournament win itself (so he could win almost the double of points than other player who won the same tournament beating only low-ranked players).
The most bonus points was awarded if you defeated the nš1 player (so the nš1 player was always at a disadvantage here, because he could not defeat himself
), then defeat of a 2-5 ranked player, then 6-10 ranked player, then 11-20 ranked player...
All these bonus points were doubled in GS respect to Masters-Series (and Master-Series with respect to International Series...).
They also changed the points awarded from year to year (a little higher year after year, or at least they upped them few different times during that decade), that is why if you watch the Year End Rankings during the 90s, you'll see that in 1990 or 1991 they seem to have lower points than in other years in that decade.
The Masters-Series or Super-9 (now M-1000) were not mandatory, so many top-players skipped some of them carelessly (they could gain almost as many points in an international-series tournament if it had a strong-field, because of the bonus points).
It was so different back then.
Another example, a very big tournament in the 90s, the Grand-Slam Cup (with great draws and probably the highest prize money) did not count to the rankings (at least until very late in the 90s decade). As I said, this was much more common during the 70s, big tournaments with big draws, where top players fought hard to win, did not count to the ATP-rankings.
Today it would be very rare to have a very big singles tournament, with all the top-players fighting hard to win it, knowing that it does not count to the ranking. But prior to 2000 it was very common.
Yet some more reasons why it is senseless trying to compare numbers from different eras.