Yields of nuclear explosions can be very hard to calculate, even using numbers as rough as in the kiloton or megaton range (much less down to the resolution of individual terajoules). Even under very controlled conditions, precise yields can be very hard to determine, and for less controlled conditions the margins of error can be quite large. For fission devices, the most precise yield value is found from “radiochemical/Fallout analysis”, that is, measuring the quantity of fission products generated, in much the same way as the chemical yield in chemical reaction products can be measured after a chemical reaction. The radiochemical analysis method was pioneered by Herbert L. Anderson.

There are many variables including how precisely the conventional explosive detonation can be timed for optimal implosion.

The 1954 test was one of the first FUSION bombs, and there are even more variables to control in that…
https://en.m.wikipedia.org/wiki/Castle_Bravo
They neglected additional nuclear processes releasing more neutrons and increasing the fission of the uranium tamper when calculating the yield.

The fission and fusion processes and the energy they release, however, is understood theoretically and experimentally.