Allow us to explain about certificates. The simple fact is all jewelers cannot sell the same diamond for the best price, but they still must make it appear that way. Thus many other certificates exist that are far from accurate, and it appears you are getting the same grade diamond, for much less money.
In the end, the real question is, just because some certificate states the diamond is a certain grade, will GIA agree? Most likely not, thus these other certificates can be discounted quite substantially, depending on how inaccurate they are. There is nothing wrong with buying a certified stone that is not GIA, as long as you understand this, and pay accordingly. It is also ESSENTIAL that you get the original certificate with the diamond, not at a later date.