472kHz ERP over-estimates?

Looking at the reports I send and receive, I wonder if several stations are actually using a LOWER ERP than they think? From measurements I am pretty sure my ERP is 5-10mW with 5mW likely to be the closer figure. Some stations claim 0.5 to 1W ERP which is considerably more than me. Some stations ARE using 6-12dB more than me and to much better antennas but, judging by reports, I should be reporting them much stronger than I do. This could be because (1) my noise floor is worse, (2) my RX is not optimally sensitive, or (3) their ERP is lower than they think. In at least some cases (3) is the reason.