BitTornado (or Shadow's Experimental or the original client) only resumes from verified chunks. If any chunk doesn't verify on resume when it re-reads the file, it discards it entirely and redownloads the chunk. Azureus saves info about the partially downloaded chunks so it can resume those somewhat reliably. The reason your download went back some percentages when being resumed in BitTornado or any bittorrent client that doesn't have or save the partial chunk download info is because there is no way to verify a partially completed chunk and the only way to be sure of it is to redownload it. You would have the same problem of downloads regressing a bit on resume if azureus lost its partial chunk download information such as by a power outage or if you delete the file Azureus uses to keep track of that info. Large percentage regressions are due to having *a lot* of partially downloaded chunks that weren't fully downloaded, which means all those partials therefore cannot be verified (to the granularity of verification the torrent file itself allows for).
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