'Shouldn't' is a bit subjective, at this point. I concede that much of all this is quite implausible- but given just how much of it is that way, and that it seems to be happening despite that, it seems more reasonable to assume that the train of logic we were using prior was based off of somewhat flawed assumptions and incomplete knowledge.

...Ah, here we are.

[However illogical the mana pool might be, he'll be able to see that Muffet did describe it's appearance pretty accurately as it comes into view.]