When an amp puts out 90 watts....

Its probably a tube amp.

That its power supply is capable of sourcing the increased current necessary to provide constant power across decreasing impedances.

As Zd says, and I just confirmed, it's a tube amp. This is how tubes amps typically spec. Nothing to get worried about. And by the way, only a very few solid state designs with massive power supplies are able to double power into halved impedances. This is NOT the norm.

"This is NOT the norm."

It's the norm for some people.

You might want to read more about this:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php

An amplifier does not have to double power to sound musical and to also have a lot of authority.