File size Vs. Total downloaded

Why my total download size (without including the data of discards) is bigger than the size of the file that am download?

Sample:

File downloading: 700 mb

Total downladed: 900 mb (including 120 mb of data of discards)

???

some of it is rubbis data

basically junk…

all clients download it sometimes…

depends on torrent and peers and seeds… and the DATA!