Could you elaborate ?
Sure, take this for instance ( from the fire dungeon pack ). 
There's 4 duplicates here, the 4 corners. Why? Not only is this confusing, but I have to edit this and rearrange it to make it usable. Here is the fixed version that I had to edit myself 
But to make things stranger, you have it correctly in other places. Like here in the winter pack 
That's how it should be, but why didn't you have the lava like that? However that is a minor issue in comparison to the tilesets with missing tiles, which are many. For instance, what's wrong with this picture? 
This is missing edge tiles, side tiles, and has a duplicate lower right corner in the upper left. In the upper right it has a center tile which is not even used with this style of tileset. And there are MANY examples of this type of thing. Howabout this one ( from water dungeon I think ) 
What the heck is this? This doesn't even make sense. All I can tell is it's animated. I have no idea how to use that. Inconsistency and missing tiles are the problem with these tilesets. Imagine your a programmer and you're sifting through these graphics and the patterns keep changing or you keep finding missing stuff. So you've got to hand edit things to make things work. Very frustrating.