Aw, Lipton, Twinings, PG and crew aren't really trying to pull a fast one or anything. It's just that people expect them to taste a certain way, so they use lots of different teas from lots of different places to obtain a consistent end product. And because teas from the same place can differ in taste from harvest to harvest, they constantly switch the ones they use in their blend. If they don't, consumers tend to go a little nutty, like the guy from Save Twinings
who insists their Earl Grey tastes different than it used to.
The History Channel's Modern Marvels
documentary on tea explains the whole blending thing much better than I can. It can be found online in its entirety here