They do it consistently as long as you see no variation over 2.5% they're doing exactly what they should be doing. The main Goxlive or wherever you get your data from is an amalgam of all the different currencies being traded at the same time.
You can look at USD only feeds if you want to be sure the data you're seeing is slightly clearer.
I was looking at clarkmoody, the chart says MTGOX USD, just assumed it was the USD feed but maybe not