Forum.ReplyCount property is giving inconsistent values

I wanted to show more Analytics using the Forums - Details Widget. Currently, if you drop that widget on a Forum, it shows the total Discussions or Questions using ThreadCount property, which is fine. I wanted to include also the Total Replies as a forum object has the property ReplyCount available.

However, I noticed that this property returns wrong values sometimes (we have the total replies for these forums as part of daily reports) and some of them don't match. We're not modifying the core of the  Forums - Details Widget. We just want to add one property, like this:

$core_v2_language.FormatString($formatString, $forum.ThreadCount.ToString()) | $forum.ReplyCount

Could this be a bug maybe? As I said, for some Forums, we get the right values, but not for others. We are using Version 11.1.12