Why is there a difference in residual stromal bed for PRK and LASIK?

I have recently been considering eye surgery to correct my myopia. My glasses prescription is -6.5 with -0.5 astigmatism in both eyes. My corneal thickness is about 494 in one eye and 516 in the other. I'm pretty sure LASIK is out of the question but just wondering about PRK? I've read that the ideal residual stromal bed for LASIK is 250 whereas for PRK it's 400. This seems like a huge difference and I was just wondering why this is?