[cocoablazing]

It's a rule-of-thumb. The fission of Uranium and Plutonium results in an incredible diversity of both isotopes and nuclear isomers.

Most of these isotopes exist on distinct decay chains. An unstable nuclear state may decay in more than one mode, each with a distinct time constant. Many of these decays occur as groups of decay chains that occur on extremely short time-scales, and may temporarily remain in metastable states (isomers).

So this general rule results from the superposition of many thousands of reactions, that proceed stochastically with distinct time constants and energy released.

There is a mathematical rule relating the time constants and energies of these reactions, but it is not simple: https://en.wikipedia.org/wiki/Fermi%27s_golden_rule

[effie]

According to the golden rule, rate constants are not directly connected to "energies of reactions", but to matrix elements $<f|H'|i>$ for different eigenfunctions i,f. The energy gap between two eigenfunctions may be very large, but the corresponding element may be zero and the rule would predict rate of transition as zero. It is only approximate rule, so the transition may still happen.