I’m not current anymore but as I recall it ANP is the level the system will guarantee (as I recall it 95% chance of being in the circle), it’s not an objective measure of how accurately the IRS are actually performing.
As you say they usually do much better. I used to cross the pond regularly with triple INS, IRS pre-GPS days and actually being 10 miles out verses a radio fix you got to the far side would be quite unusual.