Percentile rankings are solely for your information—to help you assess your relative performance. They aren’t reported to the B-schools. Even if they were, the schools would ignore them, because each B-school is interested in ranking you among only its applicants, not among all GMAT test-takers.
The overall performance of the GMAT test population has been improving gradually over the last several years. The reason for this trend has to do with the fact that the MBA degree has become increasingly popular, and that as a result the B-schools have become increasingly selective in admitting new students. The end result is that a given GMAT score doesn’t get you as far as it used to in B-school admissions. Thus test-takers are taking the GMAT test score more seriously than ever before; they’re studying harder, and therefore scoring higher.