I've seen the same behavior on my CK-20.
To get the stick to read correctly before pulling it after the machine has been sitting for a while, you can loosen the red filler cap by the seat which will allow things to equalize. When the dipstick or filler cap are allowed air to pass, things read normally.
When my HST system used to pressurize due to a clogged vent, the red filler cap always needed to be loosened first or I would get the same behavior eric describes, but that was due to pressurization, not being overfilled. The red cap would release like opening a 2 liter bottle of soda.
It wouldn't hurt to make sure the vent isn't clogged. It's behind the gas tank (about mid tank I think tucked under a mount of some type) and it's about 1/4 - 3/8" diameter tube that's like an upside down letter "J" like a gas vent for an underground gas line. There's no way to look up in there w/o removing the tank and unscrewing the vent, but you could perhaps use a piece of wire to just make sure it's clear.
Mine was clogged by a mud wasp that sealed it off. I have a post if you search for posts by me that describes the whole event... geyser of HST oil out of the dipstick and all over the dash, foot deck area, and mower deck...
Best of luck.
-Eric