It seems to me that the club have dropped a ball on the seat reservation system...apparently if tickets are bought in advance then the seats will be reserved, but if bought on the day then fans must find unreserved seating.
Previously, it's just been a free-for-all on cup games, which although annoying for ST holders who found themselves displaced, still meant everyone knew where they could and couldn't sit. I can imagine a scenario unfolding now in which some seats saying 'reserved' are reserved and some aren't (as ST holders haven't bought their ticket)...leading to non-ST holders looking for unreserved seats being repeatedly displaced.
Obviously not a major issue, but if the club want to satisfy disgruntled ST holders who want their own seat for the game, just make all the reserved seats actually reserved and anyone paying on the day has to find a seat without a sticker on it, just like they normally would.