Hi Tom,
SimBrief takes into account weight, wind, ISA temperature, and leg distance. What it does not take into account is ATC or SID/STAR restrictions that may delay the climb or require an earlier descent. It also doesn’t fully consider the impact of Cost Index on the climb distance, but the error caused by this will only be apparent at very high cost indexes. For what it’s worth, many real world flight planners also do not consider these either.
It should be noted that based on the FlightAware tracking, the flights you are simulating are very often in cruise for up to 8 minutes. This would indicate that they could quite easily climb higher than FL310 if they wanted to (assuming ATC allows and no other restrictions exist).
It’s also possible that they are quite a bit heavier than you. The SAS A320neo can seat upwards of 174 passengers, but on your flight you had only 127 passengers and no baggage. The lighter weight could also explain why SimBrief planned you higher.
In real life, on short flights like this, pilots quite often just ask for a lower final altitude than planned if they think that the cruise segment will be too short. This isn’t unrealistic, the flight plan SimBrief provides isn’t set in stone, and the fuel/time difference between FL370 and FL310 on a short flight like this is negligible.
Another very big contributor is add-on accuracy. From SimBrief’s end, the data for the A320neo is a bit limited. It’s possible it is slightly over-estimating the climb performance. From the simulator’s side, it’s very possible that the aircraft is climbing slower in the simulator than in real life. The combination of these discrepancies can add up to produce larger inaccuracies, but this is just the reality of Flight Simulation. We and other add-on developers do our best with the data we have, but it will never be quite as accurate as the real thing. 
With all that said, I will make some adjustments to SimBrief’s minimum cruise distance when selecting a flight level. Currently it’s set to 20nm, which is probably a bit too tight given the possible discrepancies described above.
Hope this helps, and best regards,