True, there's no such requirement. What I mean by "form factor" is that two Ortlieb FRs are big enough for me together and yet can be used separately - when I need them both, I can put them on both sides of the bike for load balancing, or when my wife and I go out cycling, we both can use one. Yes, I could do the same with two GT-18s, but that would be (1) too big and (2) too expensive for me. If you check the prices of the two products, you'll notice that two GT-18s is twice as expensive as two Ortlieb FRs.