yeah, that can make a difference, it's wider at the head end and it definately needs to be on correctly to fit correctly. the knot in the shockcord goes at the head end, if the ends are reversed, it won't be nearly as wide at the shoulders.
most likely it just slipped off though from movement in the hammock.
any quilt can slip off the shoulder, the narrower the quilt the more likely it is to happen. it's just one of the tradeoffs for getting a narrower (lighter) quilt. the narrower it is, the more you have to pull it off center to get coverage, the more you pull it off center the more it wants to pull back towards center.
moving around alot in your sleep makes it happen sometimes. just about any quilt can be clipped off to the pull tab on the side of a blackbird though which takes care of the issue for most folks. with the yeti, you can also rig the suspension so there's more tension on the zipper side, this also seems to work well if you don't want to have to clip anything off. a mini-biner is about all it takes though.