I still don't see the difference nor why they would give MBA stage time for these updates but not the iMac. Silent updates, for me, are for minor processor updates or alike - like they did with the rMBP.
Honestly, if they didn't already have a captive audience for the WWDC keynote, they probably wouldn't have put the new Air on stage. As it is I feel like they pre announced the Pro simply because otherwise there wasn't near enough juice in the hardware part of the presentation.
Calling a special event to talk about the expected iMac updates would be a bit rough. Remember that prior to the 2012 announcement, the last time an iMac had been on a stage was 2009.