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The way Apple set up time machine backups is very smart; instead of having lots of duplicate copies of files in the backups they do, they copy the files that have changed. For the files that haven't changed, they just link back to the copy in the previous backup (which may itself be a link back to the one before it, and so forth). This lets them fit more backups onto your disk than they could otherwise.

Unfortunately, OmniDiskSweeper can't tell those links from actual files - we ask the finder for the size of the link, and it tells us the size of the original file. That's what's causing those inflated disk usage numbers you're seeing. It also tricks OmniDiskSweeper into thinking there are far more files on the disk than are actually there, which causes the scans to take a lot longer.

If you scan the non-Time Machine folders, using
File -> Size Folder...

You'll get more accurate results (and shorter scans). You probably don't want to be deleting files from your backups anyway. ;-)