It makes sense if you assume Blizzard starts people off with a temp rating based on their QP performance, which goes up or down depending on placement performance. In all likelihood you weren't bumped down to 48 but bumped up to it.
But that doesn't actually make sense to do either, unless quick play is going to continue to influence your competitive games, it's an arbitrary distinction. It should affect who it matches you against in placement, but your placement matches should determine your rank. Ergo, your placements should be the same if you play all 10 together. Also, again, our skill levels aren't that disparate. We mostly play together, and our win rates and everything else are all basically the same.
If we queue separately and I'm lower to begin with and thus play my 10 games against worse players, and we got the same w/l record, absolutely I should be ranked lower, but once we're playing the exact same games against the exact teams it doesn't make sense to do that.
Edit: And even then, in our group I have better winrates to begin with. I'm not going to claim we literally only play together in QM, clearly not, but if we play together often, and I'm significantly worse than them (as evidenced by a 48 starting ranking vs 59-57), how would I have a better W:L in QM? It should mean when we play together it's driving my MMR up by virtue of us generally winning when together, and that thus I'm losing to better players when I'm losing, and winning against better players when I'm winning. It also then means the wins I'm getting outside of our group play should be counting LESS for MMR. None of that makes any sense when you add it up.