Ok I understand that the “User Preferences” are being changed from “Toggle” to “Deselect” accidently. That is strange indeed. I havn’t expierienced this behaviour so far. The only thing I can think of is that the “A”-Key has been defined in the “User Preferences” for different purposes.
This is the normal “User Preferences” - tab limited to the input-options containing any “A”
Note that there is no “Restore” - button present like in the next screenshot. Is this the “Reset”- option you are talking about? You can manually scroll down the list and check for every defined “A”- key and the defined mode.
Just for fun I modified every second shortcut in the second screenshot as “A”
It seems that blender is accepting double entries.
Perhaps you have a corrupted setup file with competing key definitions. Then you should reset your preferences to the factory setup.
Or it can be a bug. However other users would have run into this problem as well.