Gallery's a mixed bag. Some entries just load you into that scene and continue on as if playing, some scenes in the gallery offer no dialog at all... not to mention the names of the gallery entries sometimes offer no hint as to what scene it is, and I'm honestly not even sure if they're in any real order. It's a complete mess imho and needs to be redone properly.
Sounds like you unlocked some of the gallery without playing and unlocked other parts of the gallery by playing.
The reason why some run like the original scenes is because there is custom dialogue, etc, based upon player choices during those scenes. This was a request by many fans - including those who subscribe to the developer - because they wanted to experience the scenes with their customization.
As for the ones without dialogue, those are the ones that most likely were unlocked without playing - because since there were no variables set for customized gallery replay, nothing was included.
They are in order - they are grouped by people in the scene and then are sorted in chronological order within those groups.
The gallery is fine. The problem isn't the VN -- what it is, is PEBKAC.