it is probably a problem with the way they have their shared enviroment set up or just the continuing issue the find has with accurately determining the size of some files and/or volumes.
mine has been doing that since the day i upgraded to 10.2 and subscribed to .Mac (the same day)...Aug. 24th.
Mine does it, but only in the evenings. The most logical solution I can think of, is Apple is doing maintenance, and as they do it, some preferences aren't set right. It only happnes for a short period for me.
Maybe with all of the non-subscribers going away, Apple is going to upgrade those who do pay to have more space. This only sounds logical to me. I bet Apple will be happy not having to support 2.5 million accounts at no cost.
Each iDisk account lies on the server in such a fashion that it can quickly be expanded if a user purchases more storage space. The accounts have a virtual buffer around them. This way Apple can avoid having to juggle accounts between disks and partitions if someone decides to upgrade. As a side effect we're sometimes seeing larger "space remaining" counts than we should.