[kyp275]

Originally Posted by Esebian

Spoiler for Sort of Question regarding this Answer from Vol. 10 of the LN:

Underworld was a virtual world, but the construct was completely different from Aincrad. In this world, the strength of the imagination created by the souls— the Fluctlights, would sometimes affect the results. >>

If UW is really so different from Aincrad, how can we assume the same rules for both?

Spoiler for answer:

Eh, don't have the novel right in front of me atm, but I'm pretty sure Underworld was also based off the Seed, RATH did not create Underworld from scratch, not to mention I think it was also mentioned that Kayaba was involved with RATH at some point.

I'm not saying this is definitive, as the whole thing is still shrouded in ambiguity, but there are links present atm.