I think a lot of it has to do with a few things. First, because standard pproxies hold blocks in their memory(they only write a hard copy everyone so often), a crash can make the blocks dissapear permanently. However, this would be a problem with out RR system, and Mikas; since I belive that Dnet uses proxy software that writes everything to a HD. The second factor is people not understanding how Dnet takes care of keyspace and issuing blocks. Not all of Dnet's keyspace is open at any one time. Only a certain range(of which I don't know the number) is open at any given time, meaning that it's much easier than people think to have their randoms not count and/or their blocks not count due to being done allready as randoms. The last, and lease likely factor is the master keyserver. The MK knows what blocks were done, by whom, and possibly a few other things. Every hour, it comiles a list of the past hour, and ships it off to the statsbox. However, due to the hourly nature of the MK, it might be possible that it may have some inadvertant holes in the logs it sends, where a fraction of time is missed. However, since that's a very unlikely situation, I'm willing to bet it's the first 2 that are the causes.