How do you explain why the A/S increases on thermal entry?

If you have the stick in a fixed position, this translates to a fixed
AOA. If you move from still or sinking air into lift, your AOA will go
up momentarily. Assuming you do nothing with the stick, the aircraft
will seek and return to its configured AOA, which will result in a
slight pitch down of the nose and a slight increase in speed.

Said another way, the increased angle of attack also affects the
horizontal stabilizer, which mometarily produces more lift, pitching
the nose over slightly, with resulting increase in speed.