Is there some law of physics I missed in school? This may be sorta true
in some cases, with some amps but it's far from a universal conversion
factor. I can think of many circumstances with my own gear collection
where this is absolutely not true.
The missing bit is distortion. The reason the tube amp can seem to play louder than its watts suggest is that they have much smoother clipping character- rounding the output waveform when clipping rather than chopping it off as a solid state amp will do. So you may not hear the clipping at first but with a solid state amp you will.
As the tube amp approaches clipping it will generate more higher ordered harmonics. Since the ear uses these harmonics to sense sound pressure, it will sound louder at that point but a sound pressure meter will reveal what is really going on.