What is an effective seat angle, anyway?

Look at the diagram. This is the frame geometry illustration from the YT Tues DH bike, but almost any frame will do.
The red line displays the actual angle that the seat post will sit on. But notice that it runs well in front of the bottom bracket of the frame. Seat angles are measured from the centre of the bottom bracket, not from some random point in space.

Screen Shot 2020-05-12 at 9.27.48 pm
Turn your attention to the purple and gold lines. The purple line is the effective seat angle if you have a low seat height. The gold line is the effective seat angle with a higher seat height. Due to the actual angle being so slack, every increment of extra seat height pushes the seat back much further than one might expect for whatever measured effective seat angle was at before the seat went up.
There is a significant two(ish) degrees difference between the purple and gold lines.

The latest bikes accommodate this effect by using a steeper actual seat angle on each larger frame size with lots of knowledge about how long the seatpost is likely to be on each successive increase in frame size.

This is rarely even an issue on road bikes because the seat tube actually is a straight structure aimed at the bottom bracket (such as all bikes were when they constituted individually joined metal tubes). With full suspension, it is increasingly an issue for MTB. Seat tubes are complex, S-shaped structures that pick up the upper suspension pivot point on their way from the bottom bracket to the seatpost. They could conceivably point in any direction unrelated to the traditional measurement methods.

Once you know what's going on you should understand why this can be a problem for very tall (or very short) riders on a given size.