Was there a lot more liquid in the foil when you unwrapped the second half later? Bet there was. I don't believe it's guessing at all. The rest period, on a whole piece of meat that's wrapped immediately, has the surface intact, and the moisture is re-absorbed by the meat. Once you cut into the meat, you create an avenue for the juice to flow back out of the meat (an "open wound," I suppose). Even though you wrapped the second piece, it had an open end, and I bet a lot of juice flowed into the foil.