Oddly, it took many stations and programmers decades to realize that there was no such thing as "Top 40".
Call-out research on currents that developed in the later 70's really showed that there were less than 20 real hits at any given moment in time.
Yes, there was new music... as many as 5 or 6 new cuts a week... that Top 40 stations played. About half, at best, became real hits. And there were songs that were fading; we now call them recurrents. Those could be played more lightly, as they had that crispy feel of getting burnt out and turning negative unless we slowed them down.
So, really, if you add the 20 currents, an average of 5 "new" songs and maybe 10 or 15 recurrents, you do have 40 songs. But half or less are proven, current hits. Just looking at those station charts or "surveys" proves that.
Also worth noting that, since stations only had a few rotation categories for all their currents, there was no difference between, let's say, #13 and #18 or #6 and #9. So most of those charts were very arbitrary and done more to amuse listeners than to reflect any real data.