They're similar lines and were released at the same time. Was it just forgotten?

Nope, it was done on purpose.

That.. is odd. What is the difference between them?

ETA: Thanks for letting me know it wasn't an oversight, btw. :)

Probably because of those two items from both sets being extremely useful against the more difficult 1 player challengers. And in general. :P

I don't remember any of the battlers being as impressed with the Azathoth items' stats as much as with the two cuirasses.

Probably not enough Azathoth in circulation.

What? Pray tell how you came to that conclusion. I'm highly interested.