I have never noticed the discrepancies in this bit of scripture. I guess, like the disciples, I wasn't paying enough attention, especially to this very interesting point about "one loaf" being in the boat.
Mark begins by noting the seemingly irrelevant detail that the disciples had forgtten to bring bread and had only one loaf with them. On one level, this simply means that they have failed to replenish their food supplies. But the two miraculous feedings and the ensuing discussions have prepared us to understand: What is the real "one loaf" (literally, "one bread") with them in the boat? It is Jesus! Mark clarifies in verse 16 that actually they have "no bread"--no earthly bread, that is. Bread will not be mentioned again in the Gospel until Jesus announces that the bread is his own body, to be given up for us on the cross (14:22).