The first thin that comes to mind is the aircraft that will be used will be a Thomsonfly 767 that has been reconfigured into Tuifly Nordic configuration. It's the Scandinavian version of Thomson for which Thomson airways do a lot of work for in the winter. Their back cabin(row35-47) is economy configuration. Rows 7-33 are normal 33" same as usual and rows 1-5 are premium (row 6 has 40") because of the different configuration that might be why it's not in the System yet.
Alternatively it could be they know they don't have time to reconfigure an economy 767 to long haul specs and they know at this stage they will be using one of those.
To be safe pick the middle cabin if your travelling in economy, if you then upgrade to premium then that gets taken off your upgrade price/refunded.