Up2Vex I think that's because the extension doesn't count the first post in a discussion as a post (eg.: when there are 5 discussions and 0 posts, then it's wrong and it should have 5 posts instead). It can be confusing sometimes, as the statistics widget extension you are using seems to get the total post count right.
As we do not maintain those extensions, please kindly ask the authors for a fix (you can do that by searching for their discussions here, on Discuss). Keep in mind that this might also be intended, if Flarum treats only replies to a discussion as an actual post (or the categories extension).
