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This is actually wrong

, icedout921 have explained it correctly.
Ichigo have indeed become stronger by training a few months in jinzen, but he didn't defeat Fusion Zangetsu and we don't really know how much stronger he became, because the moment we see him again (after accepting his Power), Ichigo's Zan has already his limiter removed off (and thus this itself could boost his power) - it looks that away at least to me.
If you have played some fighting, or action-platform-rpg games (for example Ys Origin), you could say that the moment he appeared in SS, he was already in boost mode, where his boost gauge was depleting slowly, but the moment he casted FGT - Mugetsu, he depleted it entirely.
Now the most important part, after returning to earth through the tunnel world, the time he spent in it and the training he has done, have been reversed to the state before entering it. That's what Kisuke told the gang (and the gang told Ichigo), but who knows, it might have been another plot-but in reality Urahara lying again-twist.
Thus, Ichigo Shinigami x Fullbring, is already stronger than what he should be, if he had only regained his Shinigami power (because the Fullbring part was always the unused power that Ichigo always had). This as well is the reason why the gang doesn't seem to have trained/powered up much, since Ichigo himself got stronger as well (in a way ;P).
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