Yeah, I remember some weird results if digging deep enough in the performance manuals. One of them was that 3/4" slush would allow for a higher weight than 1/2". Forgot how but it made sense when the instructor explained it.
I have noticed the same. Is it the roll resistance from the slush/snow that helps achieving a better deceleration if stopping, hence you can take more weight? That's the only reason I've come up with so far.