Some time ago, I explained why the mouse cursor jumps a few pixels if you click on the very bottom row of pixels on the taskbar. Commenter ender noted that a similar jump occurs if you right-click the Start button. What's up with that?
This is not an issue with Fitts's Law, because you already hit the target successfully. The purpose of the little mouse jump is to put the mouse into the menu. I guess the idea is that the people who designed that menu wanted to enable the mouse shortcut of right-click followed by left-click. When your taskbar is positioned at the bottom of the screen, this means that a right-click followed by left-click invokes the bottom menu item, which is normally Desktop.
Bonus chatter: The Win+X menu was a secret feature, which isn't quite the same as a rogue feature. The feature was reviewed and approved, but it wasn't something that was advertised. It was a bonus thing for power users. Allowing the Win+X menu to be customized was also intentional, and it's a tricky balancing act to come up with a design that lets the menu be customized by the end user without making it too easy for applications to customize it as part of their installation. (There were a lot of designs that had to be abandoned. For example, we couldn't incorporate the user's SID in the hash due to the possibility that the user's SID changes.)