so that's supposed to be an avatar of a Numenorean eh?...looks more like a Nazgul to me! (and yes, I've read The Silmarillion)

anyways, it probably because the SHA1 hash value for the file you are resuming with is different..i.e. its not the same file...hence, it is basically starting over again...very annoying, as I well know...the best advice to avoid this setch is to make all possible attempts to allow the download to complete as opposed to logging off and hoping for a resumption