What I'm trying to say is that you get used to checking that achieved EPR = commanded EPR, not that the numbers make sense on their own. If the commanded EPR is *grossly* wrong (full derate where there should be hardly any, as above), it doesn't stand out in the same way it would with N1 as the controlling parameter. Yes, it's a good habit to look at the N1 as well as the EPR but when we're tired or in a hurry, "computer says OK" often ticks the mental boxes: all those needles lining up with the little bugs, quite compelling.
If you were in a Cessna and got 2200rpm instead of 2500rpm, you'd know there was a problem and reject the takeoff. If the gauge read 1500 one day and 3000 the next, for the same power, it wouldn't be so clear cut.