Most likely because they're also downlding from other hosts at the same time. So whilst they might get some chunks of the file from you, they're also getting chunks from a no. of other people. Probably when a chunk has completed, there is a break before communication is sent to get the next one that they're after that they haven't got from someone else.

shaunpc I'm not sure why that's the case. I have noticed LW 4.10 has more sensitive speed measurements. I don't know whether that extra bit rate is overhead or what. I've also noticed similar inaccuracies in the speeds shown in LW.