[jlittel]

Discarding losses on time could plausibly be the bulk of the effect.

I can improve my chess rating in short time controls by a good 50+ points just by reminding myself to move quickly.

[agarden]

True. Also, people tend to take more time when in losing positions, so it is likely that more of the discarded games than the not-discarded games would have been lost.

[lumpypua]

It would be very interesting to see a computer analysis of position strength in the discarded games and seeing, where possible, if the ratios of "likely win" and "likely loss" match the games that were finished.