It appears that revisions are indexed starting both at 0 and 1 (related, sometimes they just don't make sense at all).
For example, this question says "9 revs, 8 users". But in the revision history, the most recent revision is numbered 10 not 9. That 10th revision is also a reversion, to revision 8 (the summary is automatically generated).
The original post was not a revision of itself, obviously, so 0-indexing and "9 revisions" makes sense. But it was the first version, so 1-indexing and "10 versions" makes sense as well. But I think we should have something consistent so we all know what we're talking about.
Solution 1: Have the revisions page start the numbering at 0, or have the original post marked "OP" and the revisions to it start as 1. (The auto-generated text on a reversion would follow suit.) This is my preferred solution, since it makes the numbers consistent between versions and revisions.
Solution 2: Change the text on the posts from "X revs" to "ver. Y". The example question would then show "ver. 10, 8 users" rather than "9 revs, 8 users". I prefer "revs" though, and hence prefer the first proposal.