apt-get update issues with apt-cacher

[Tomoki Taniguchi]

I have an apt-cacher setup to reduce the amount of
bandwidth used by all my ubuntu machines.

but i have noticed that since setting up my machines' sources.list
files to use the apt-cacher, "apt-get update" seems to be regetting
ALL the package lists not just the one that have changed.

how do i fix this so only the updated files are downloaded?

TIA,
Tomoki