Been like that for a month or two now - always too high.
Putting this on SR Meta rather than SE meta because it only happens on this site.
Happening to anyone else? Resetting cache doesn't help.
This is a known bug: the counter at the top includes all open reviews, not just the ones you can act on. This includes the reviews that you've already acted on, as well as the occasional review you aren't eligible for in a queue that you do have the privilege for (e.g. a tag wiki suggested edit that you submitted).
This bug exists on every site, but you'll only see it often if you have the “10k tools” privilege (2000 reputation on beta sites, 10k rep on graduated sites). For lower-reputation users, the indicator in the top bar only counts pending suggested edits, and this one does subtract the reviews that you've already acted on. It's only high-reputation users who see an indicator for all reviews, which doesn't take already-reviewed items into account, which is usually out of synch.
See the following threads and more on the main meta: