So what? What caused coal to begin with?
" Most of our coal was formed ... years ago, when much of the earth was covered by steamy swamps."
http://www.ket.org/Trips/Coal/AGSMM/agsmmhow.html
That seems to indicate that coal was formed in a certain period. The period was a time when a lot of the earth was swampy. That means inhospitable to most plants and creatures from Eden. Remember, that Eden was the hospitable part of the earth. Only certain plants would have ended up there. Those plants likely were, if the coal forming period was early enough, some plants that were created for the earth at large. (as I say, most were in Eden), but it seems some were made for the earth at large as well, some creatures and plants, helped to get the earth ready for us. Now, we may add to that pollen or perhaps some seeds birds dropped. That depends on how long the swampy areas were there. A point comes when most coal is formed, and locks in the fossils of plants it had at the time.
Later, more plants arrive in the area from Eden's migration, on layers above that. Naturally, only some plants would be reflected in the coal record.
Piece of cake.