Sandra that is an excellent question that I'm sure many have wondered. I would guess to some degree genetic as noted. For me it seems to also be wrapped up in the health of the plant. I have a few phals that are totally fine in terms of roots, but I purchased them as rescue plants and they seem to struggle to maintain 2-4 leaves for several years. They just cannot seem to get "over the hump". On the other hand, I have a couple of others at the opposite extreme - one even a mini phal type - with 10 leaves. As Cindy mentioned, it is awesome to see truly mature phals with leaves that are 12 inches or more long each and 4-6 inches wide and it seems like about 3/16 inch thick. Most of us have not been growing phals long enough to have one of these for our very own. Maybe someday?
