Soft sided bags are pliable and stretch. Therefore, then can hold more
than the dimensions.
But if the outside dimensions are right at the regulation size for a carryon, then over-filling it and bulging the sides will probably mean that the bag will not fit in the sizer. In this country, the regulation size for a carryon is 22" x 14" x 9", or 2772 cu in (45½ L). If you try to fill it to more than that volume, the bulging side will probably be cause for rejection in the sizer. And remember, just the nylon cloth that bags a soft bag probably occupies 100-200 cu in, so the actual capacity is less the 44 L.
Do you suppose the 65L in the name of the bag (Osprey Fairpoint 65L)
means 65 liters?
Yes, that is probably what it means, but the overall dimensions of the Fairpoint 65, 70 x 41 x 34 cm gives an outside envelope of 97.6 L. But it's not a rectangular solid, and the volume lost to bulging pockets, the wheels, and the handle tunnel takes away from the outside envelope. I would assume that the 65 L is an honest capacity calculated as Frank describes above.