Beef, it's a combination of two things, first, as Rick pointed out and as Ian and I have discussed here in the past, +/- 3dB means a 6dB "window" or "swing", not a 3dB down point. Looking at the uncorrected graph, the M60 would be about 6dB below the 90dB high point(at around 95Hz)at 55Hz. Then, the low frequency correction Ian discusses in John B's link apparently shows that the 6dB down point measured anechoically is actually at 37Hz rather than 55Hz. Of course, in-room reinforcement changes the real-world result.
