Mine is a 20, Jeff. Maybe a prior owner stuck the 15 amp spare in where a 20 should be, and that's why the circuit can't handle your icemaker?
Addendum: Oops. My bad. You're right, that circuit is a 15 on mine also. The ID tags on mine are skewed; it's position #6, not #5, and there is no spare in position 8.
If there is an ignition relay where I think, in the icemaker outlet, it might be configured to cut the circuit (IC6 for I assume "Inverter Circuit #6" off the 6th Inverter [30 amp sub] Box breaker) downstream from that outlet (the 2 A/V and the 1 TV outlets), but not the icemaker outlet itself. Just ruminating... always a dangerous pastime.
Joel