Shouldn't black holes masses add when they collide?

[whitesphere]

I may be wrong, but I assumed when 2 black holes collide, the result is a black hole with a larger mass.  When they collide in the current Universe Sandbox, the result is one black hole explodes, its mass apparently disappears, or is converted with 100% efficiency to energy.

Or is the mass added and just the black hole's label isn't changed?

In any event, Universe Sandbox is a very interesting application.

[Physics_Hacker]

The label isn't changed-it doesn't do that on it's own.