Hello!
All SR/LX-A319/A320/A321 have convertible seat rows for C-Class. This European C-Class-Product was introduced in the mid-90's with the delivery of the new small Airbusses (I remember that also one MD-81 for the ZRH-VIE-Shuttle was equipped with convertible seat-rows).
But there is one exception: To obtain a higher capacity, the Airbusses on the pendulair-flights ZRH-GVA are always equipped with 3+3-rows in both classes.
I guess is that what you have seen.
Greetings from MUC
Patrick