Yes! The 21" mowers are able to sense when the blade encounters resistance and increase power to maintain rotation speed. Think of it as riding the throttle on a gas mower, starting on medium and increasing whenever you hear the motor struggle.
There is a brief delay in the power adjustment, so the blade may slow a bit when you first hit a thick patch, then rev up to get the blade back to full speed. I believe when you reach the end of the thick grass and have less resistance the extra power makes the blade speed up briefly before the power drops back down. You can definitely hear the motor get louder and softer as the power needs fluctuate while mowing.
This is INFINITELY better than the 20" which would sense the extra resistance and the blade would slow down, reducing cutting performance and flashing an overload warning light, possibly even shutting off if the grass was thick enough.