To complicate matters again....
Different saddles allow more or less fore-aft adjustment, and that goes for seat posts as well. Older saddle designs, like Brooks, tend to assume a seat post angle around 73°. If the frame builder makes the seat tube angle steeper, most riders will want to move the seat back to compensate... which is often hard to do.
I made a post a bit earlier, that vanished. I will expand on:
Seat tube angle, WAS directions from the builder to where to place the saddle, or their world view on where the saddle should be placed, by whatever thinking was at play.
Often that can be worked around. (Way back that was done with "7" shaped seat posts. Never really understood that- why not take a direct route?, but I digress)
On very small frames the distortion of seat tube angle was done to "make the numbers" of top tube length most often, sometimes coupled with a slack head angle to help the top tube measure and provide a UCI acceptable front center dimension. Not sure about the recent versions of the UCI rules on dimensions of bikes but there are a number of key ones from decades back, max bottom bracket height, wheel base dimensions (to prevent recumbents), front "center" dimensions to essentially prevent "toe clip overlap". Race bikes were type formed and this infected almost all. I also think the UCI has an issue with short people.