The 18:02 IC to Zürich (which I assume is what the OP took) is a proper IC train, not an IR. It is normally composed of a standard 9 car double deck rake (of which 3 are 1st class) reinforced with a 4 car single deck "module"(and an extra locomotive). The 1st class cars are always in sector A in Bern for trains to Zürich. The "module" that is added during rush hour also has one 1st class car, and this one would be in sector C. I think the OP just missed that most 1st class cars were in sector A, and waited in sector C where the single first class car was full...
One tip: If you do not yet know the SBB App, get it. In the app you can click on the train details, and then open a menu (by touching the three dots in the upper right) where you can see the train composition at the stations of interest to you.