Maybe that explains it a bit:

When LW downloads a file, it first ask the host about the identity of the file, known as Tiger Trees. Each file has many identifies, one for each fragment of the file. The size of the fragment varies wheter the file is big or not. For a mp3 it might me 128KB while for a movie it can be 1MB or 2MB. That's why you see the tiger tree hashing transfer often in the upload window before uploading a file.

When you finish downloading a fragment of the file LW will compute the identity (hash) of that fragment with Tiger hashing. If it does not correspond, LW will delete the fragment and try to redownload it. That is why you can see your dl percentage going backward. If you want to see how much data is corrupted in a download or simply the size of the fragments LW use for one file, enable tool tips in the advanced options while righyt clicking on the title column in the download window.

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