I bought an 11-28 8 speed cassette for my Ultegra equipped road bike, the shop said it should work even though it's labelled as an mtb cassette. When I put it on, there was play so the cogs moved side to side. An idea struck me, so I put the 12 and 13 from the stock cassette (12-23) on and ditched the 11 and 12. Not only did it eliminate the play and tune up fine, but I wanted a 12-28 anyway. But why did that work so well?