Themes and ROMS?

Can anyone explain to me why some themes don't work with some ROMs and others do? If they all manipulate the same files I would figure they should work.

thisis due to changes in most roms

They all have slight differences in framework and some may have files in different folders, or like the MOst of the Froyo roms they changed file names alot.

Im sure there's more to it than just this but for the most part thats the biggest changes that cause some themes to not work correctly on some roms.

My Best Explanation:

A ROM is like a dog. All dogs are different lengths and sizes. A theme is like a silly sweater that grandma knits for the dog. Now, each sweater needs to be knit to fit the dog it was made for. If you try to put the big sweater on the small dog, it might fit but it wont work quite right... and if you try to put the small sweater on the big fat dog, it won't work at all!