User blog comment:WhiteArmor/Ichigo-Teichou?/@comment-1743015-20100410015530

Two things: 1. Isshin was a captain before Urahara and it could've been awhile before the TBTP arc when Isshin "died". 2. Shinji could've recognized it, but just didnt say anything. Or, maybe its been so long and Isshin's changed so much that Shinji doesnt recognize him anymore.