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

It's useful to look at extremes here.

If a plane was going straight down (extreme case) and flew into a 10
knot thermal, it's speed would increase by 10 knots instantaneously.

If a plane was going straight and level, and flew into a thermal, it's
speed would increase by zero knots. no increase.

For any rate of descent, the plane's speed would increase upon entry
into a thermal by some value between 0 and 10 knots, varying based on
it's rate of descent/angle when it penetrates the thermal.