I would imagine that someone just stuck an SE/30 ROM in there because they saw an empty ROM slot. Some Macs won't boot without a ROM SIMM, that may have been their logic in the matter.

It could very well be that the Quadra 700 recognizes that this is an SE/30 SIMM and chooses to ignore it, or maybe there is a jumper (possibly hidden) to even enable the ROM slot, similar to the IIci.