The calculated MECPs heights and SOCCs reveal that the different effect of methyl group on the ISC rates in the double-bond compounds versus the triple-bond ones can be attributed to different mechanisms. In the former, the MECP is very low and the rate is controlled by the SOCC variations, whereas in the latter the MECP is high and the main effect due to methyl substitutions is explained by the variations in MECP heights. Our study also highlights the great complexity of these small species, drawing attention to the methodological challenges and possible pitfalls in theoretical modeling of their reactivity.