They hafta say that, coz they know that if the test of a true prophet was applied to them (as in Deut. 18:22 and Jer. 28:9), they would be exposed for what they really are - false prophets. So they'd rather you didn't make such a big deal about all the times they get it wrong, which is pretty much all the time. The jury is still out on maybe a couple of them, but if I were a betting man........