I believe it primarily happens when you review posts that are contained in multiple queues. For example, a new user makes a very low quality first post. Our site is low volume enough for that effect to be significant.
This is by design.
Please read the canonical meta.see post:
What does the number next to the review link in the top bar actually mean?
The number is those outstanding overall, not necessarily those awaiting your input.
It does seem a bit odd but don't worry about it :-)
The number you need to have earned on this site excludes the joining bonus, as it was earned from other sites.
So you have 3 earned on this site - not enough yet.
In order to improve your rep while you cannot answer, try improving your existing posts.
I can't post this as a comment, so I am posting as an answer. I had not upvoted his answer as of reading this, so I opened IE 11, logged in, visited the URL, opened the dev tools, then upvoted.
The vote went through w/o an error.
The Request was sent to /posts/14990/vote/2 and the body was populated with (obv I have corrupted the key with the F's):