Looks like the same clam to me. Your photo shows it a bit more open. Totally understand that you paid for something specific. But they are live animals too and nothing about a coral stays the same I'm learning.
I skimmed through the pages, but looking at both pics, it appears to be the same clam to me. Their pic is when that clam just arrived, so the shell is going to be nice and clean. Also, it's not quite fully open, so you can't see all of the mantle. Your clam is the same, but now it has algae on the shell and maybe the color has changed slightly, but is still pretty nice. Colors will and do shift based on several factors, so expect that going forward. My guess is that the clam was in their facility for several weeks before you bought it.