Decreasing amount of uploaded bytes

[CyberMonk Titan]

As part of scientific research I've been running utorrent via automated scripts on several servers to measure the performance of the client under several circumstances. I log the progress of a torrent's download/upload using the web API: every second I send a request for data and write that to a file, including timestamp.

This all works quite fine, I get nice results and the client is pretty fast (as expected).

However: one thing is bugging me: I get negative calculated upload speeds. Easy enough to ignore, but still a bad thing.

Let me explain: the upload speed as reported by uTorrent itself is unreliable due to possible high frequency of change (for one particular session the calculated total download size from the reported speeds integrated over time was about 180Mb, after it had completely downloaded a 500M file...). Therefor I use the downloaded and uploaded statistics: the number of bytes received and sent. Subtract and you have a nice speed approximation.

Without further ado a few relevant lines of the log I took from one session:

1322560883.561916188

["98C8FED27E6674E42FCDD55EB7AAA6380FA776EA",201,"file500M",524288000,1000,524288000,91209728,173,263480,1091,0,"",1,5,0,13,128974,-1,0,"","","Seeding","cd4ba5c9",1322560819,1322560847,"","/local/tschaap/tmp.68aJIvoyVj/clients/leecher/exec_15"]]

1322560884.574092069

["98C8FED27E6674E42FCDD55EB7AAA6380FA776EA",201,"file500M",524288000,1000,524288000,90685440,172,210989,1027,0,"",1,5,0,13,129433,-1,0,"","","Seeding","cd4ba5c9",1322560819,1322560847,"","/local/tschaap/tmp.68aJIvoyVj/clients/leecher/exec_15"]]

So that the output from the same client during the same run on the same file (in fact: there is only one file in that client and only one client running on that machine).

The line with just a number is a timestamp in seconds, the second line is the reported status of the torrent at that time. Notice the seventh entry in both lines: 91209728 and 90685440, respectively. That entry is documented to be the total number of bytes uploaded for this file. So in this case at time T 91209728 bytes had been uploaded and at time T+1 90685440 had been uploaded.

The error is evident: the number of bytes uploaded has suddenly decreased. It seems to me this is quite impossible: I don't usually 'unsend' bytes to a network.

Is this a bug that occurs somewhere? Or am I misinterpreting my results?