Just thinking out loud here but it may be something to do with different devices used for the SCR and Q3 and their characteristics.

Just as the oscillator circuit provides induction to a high voltage through the HV transformer, so transient voltages from the HV discharge can pass back the other way. This may have a beneficial or a degrading effect depending on when this occurs. i.e. if the phasing is correct it could tend to turn the Q3 transistor off for a short period during firing. If not, it might be possible to turn Q3 on harder.

SCR's are also susceptible to "rate of voltage rise". This can actually turn them on if excessive. Usually selecting a higher voltage SCR can be beneficial.