User blog comment:Xilinoc/Ch. 532 Everything but the Rain - The White Noise/@comment-823339-20130403195544

I think Isshin after regaining his powers (aka Fake Karakura town arc Isshin/current Isshin) might actually be stronger than flashback Isshin. If Isshin learned Final Getsuga at some point between this flashback and the start of Bleach, it's quite probable he must have been very, very powerful at the time of using it (the same way Ichigo was - not to the same extent, but still experiencing a power-up). After he regained his powers, he could have regained some of his experience and power from the time he used Final Getsuga (since using it requires mastering one's zanpakuto), which means his current strength is the result of the Final Getsuga training and not simply his previous basic power.