Yes, "stale" browser cache is a good explanation of the effects actually described by the OP.
I still think you got me wrong - the inconsistency I was referring to is when node XP goes from "-1 (+0 -1)" to "0" then "1" and that is only inconsistent if you contrast it against plain counting of all up- and downvotes. My fault if I did not state my thoughts clear enough :)
Inconsistency does not have to be a bug if it is there for a good reason and I think there are enough good reasons around.

Regards,
jonix