ubuntu update manager bug in showing download percentage

[MrAli]

sometimes when i use ubuntu Update Manager to update my ubuntu 12.04.1, it does not show correct progress percentage of downloading package.

for example i tried to update kernel (with 38 MB size) but even when i had downloaded 35 MB of file, Update Manager had showed 56% progress for downloading the package.

i should add that this problem does not affect on downloading of packages and even in my explained example, when it recieved all the 38 MB, the progressbar and percentage both jump from 56% to 100% and the package installed correctly.
here it is an screenshot about this problem:
http://upit.cc/i/ea633d9f.png