Diff from middlecoin increased from 512 to 1024. That's why. That number doesn't mean anything but a stat that you can compare to itself and only when diff stays the same
Yeah, that would be true for a pool mining a single coin for many hours straight.
I don't think that middlecoin really switches 1500 times a day as someone guesstimated a few pages back. But with frequent coin switching and below average performance rig (sub 500 kh/s) your chances of getting a share accepted before the pool moves on to the next block or switch to another coin occurs are lower with 1024 diff.