Actually you may be on to something there. It could well be, and should be, the case that the belts are required to a higher FoS than the steel bars. The belts are a consumable made of materials with a high rate of degradation in UV and are also subject to fraying and wear along with strength loss just due to the wrapping and buckle arrangements. Having difference FoS is no different than in the crane industry where the cables use 5:1 FoS on BS versus 1.25 x YS on steel, now in crash situations things ar euseagly not designed to an allowable working stress but rather to some ultimate point just below total collapse. Their even mild steel will typically not fail below 100 ksi.Actually, this is what I am assuming the difference probably is. The problem is that according to both of the posted regulations, the 3K load is for the shoulder belts explicitly, and there is a separate load for the lap belt.
It seems like a lot of this may be based on SCCA roll cage specs, which do specify a 1.5" harness bar, but I would like to see what loading was in their design, as it doesn't jive with the 3K requirements. It is easy to believe that they are using something similar to what you just said, but that would not be very good communication between them and SFI if the belts are designed 10x stronger than what they are attached to.