It just doesn't make sense, the Talking House 5 and the iAM are identical units in every respect with exception to an audio enhancement component that's added for an additional $100. Maybe you got a bum unit. Perhaps you should contact customer service.
It's also worth mentioning that the HB review (www.hobbybroadcaster.net/reviews/iA.M.Radio-AM-Transmitter.php) commented "..To our surprise, the coverage was slightly better than our documented coverage with the Rangemaster mounted in the same location. Coverage was slightly better than three-quarters of a mile..." (That's with the ATU and a 100% compliant ground mounted install.
But my major point is that the two models are essentially the same unit, so naturally should perform identically.