Have got to the bottom of it (with my company anyway - can't speak for any others!)
We sell flight only which is called a "1 night explorer" ie your documents say you get 1 nights accom with it.
The dormhouse is (or should be) £10 dearer because in theory, if you wanted to, you could expect accom to be available to you for your full duration. So it's bit dearer as you are paying for accom that you will never use.
So go ahead and book the cheaper flight only price!! (If you are booking that is)
I went to Goa twice earlier this year with Mytravel from MAN, I was only asked the first time to prove I had accom (I had a package booked so all ok) but wasn't asked the second time. Seems a bit random!
Anyway hope that helps, if not give me a shout!