I did some more testing ...
I can simply the issue now.
When I create an event using the Google Calendar web interface, iPhones do get these events as invitations and not as events as such. This was never the case.
Anyone else using Google Calendar with iCal and/or iPhone?
It would help already if I would know that other people don't have this effect.
BTW ... I just noticed that the effect doesn't show on iPad (iOS 6.0.0). Both affected iPhones have 6.0.1. Is it iOS version related?