How Do iTunes Update Their Charts?

I have to check iTunes charts quite regularly and for some reason the charts i see on iTunes are not necessarily the most up to date ones.

I know this because my colleague also runs iTunes and the Charts he gets are sometimes (not always) more up to date. Is this something to do with my IP address so that the charts are kind of cached? Even when i close iTunes and restart it the charts shown aren't necessarily updated. Is there a way i can ensure that i'm always looking at the most up to date chart ?

Thanks in advance for any help you give