I've witnessed the same problem in SSRS 2005 Report Manager, too. The report is designed and displayed perfectly in BIDS, but fails to render the same in Report Manager. Viewing the report in the .net viewer seems to be ok, though.

So, I don't have a solution to the problem. All I ever seem to do is flip back-n-forth between what I designed and how it shows up in report manager... repeat until the display in report manager is "good enough." A crappy, wasteful way to work.

There must be documentation somewhere that explains the rendering differences...