The mass equivalent of kinetic energy. Energy and mass are equivalent; if you know one, you know the other through E=mc2. In other words, perhaps what is being converted to gravitational waves is the kinetic energy of the system (not its mass, per se) but which, of course, can nevertheless be expressed in solar masses. I was reading elsewhere on the net someone who said a star the size of the sun, which is flying through space at 40% the speed of light is packing with it the equivalent of one solar mass worth of kinetic energy. So that's what is going on: The kinetic energy of these two merging black holes is equivalent to the mass energy of three of our suns. And that kinetic energy, not the mass, is what is being released as gravitational waves.
Now, I know your next question will probably be, "Yes, but, how does kinetic energy get converted into gravitational waves?" LOL, don't ask! This was hard enough to come up with. :-)
Remember, I'm not a physicist. Just a math major who took some physics. And not enough either. I wish I had taken a lot more.
Normally, in my experience kinetic energy is converted to heat or deformation of an object.
So, I would think that the merging black holes would create heat and crush each other during their merger.
But since black holes are exotic mater, I can see that your explanation could be correct.
Thanks for the explanation.