On Fri, 25 Jan 2013 09:32:46 -0500, Tomsic wrote:

If the desk lamp also has a UL or CSA sticker, then a 60 watt
bulb was used to test and approve the fixture for electrical
and fire safety. Using a 75 watt bulb voids that listing.

Makes sense. But doesn't answer the question.

Seems to me bridges are designed to hold twice what they
say they can hold. Elevators are the same. A room placard that
says a ballroom can legally hold 100 people can 'fit' twice
that easily. Speed limits are set but we routinely go twice
the speed limit safely (maybe not twice - but the point is the
same). A rope rated for 100 pounds can handle ten times that.
etc.

It seems, to me, a lamp rated at 60 watts must have been
tested at twice that (or some large number like that) in order
to get the rating.

At least that's how 'other' ratings seem to be done.