Is that so? Tell me, which "tests vary?"
Does the WAIS vary with the Stanford-Binet? Or does Mensa use astrology to measure IQ?
Yes indeed. They all have different (slightly in some cases) scales so the only way to compare is to go by the percentile for a given score for that test.
Saying something like "my IQ is 123" is meaningless unless the test used is known. This is a popular misconception about IQ scores.