How to figure output from efficiency in db ?

I always understood that doubling power in watts added 3 db to the speakers output in db. And that 10 times more watts caused a 10db increase. Thus, if your speakers have an efficiency of, say, 90 db/watt, a 2 watt signal produces a 93db output, a 10 watt input would provide a 100db output, and a 100 watt input would produce a 110db output (all other factors being equal).
Lately I have seen, on the Musical Fidelity website and many places elsewhere, the claim that, for example, a 100 watt increase causes only a 13db increase. How can this be?
Isn't the formula still the one I have been using?
(This has nothing to do with allowances for distance, room conditions, etc. This is entirely a matter of their using some other formula.)
Can anyone clear this up for me?