recreation steps:
SBLGNT gives the expected result of 5696 but I can reproduce your count of 5674 for the UBS4.
lang Greek x field Surface Text gives exactly double at 5678
This happens when you have a group of words that contains a mix of Bible and non-Bible text (for example, ΜΑΘΘΑΙΟΝ). The facet counts are showing how many of the current groups contain Bible text without considering that the count might change, preventing it from being displayed when the facet is applied.
I've written up a case for this.