There's an optimum altitude for any longer distance flight, and it depends on the aircraft, how heavy it is, and the winds aloft. The heavy widebodies you mention, especially the 744, can get high up if light but if heavy the optimum altitude will be nearer 35000ft. The A350 has a much more efficient wing so it may well be able to get higher, and it's a particularly long-range aircraft so on a route where others are flying 744s the A350 may not be at max weight. Maybe the A350 is at max zero fuel weight, but not needing to also take a maximum load of fuel on top of that ZFW weight, resulting in the takeoff weight (and then later weight in cruise) being a lower proportion of maximum takeoff weight so the aircraft can fly higher.

These days Cost Index (which is roughly "efficiency of flight") is everything, and unless constrained by routes, ATC, etc, any sane carrier flies at the lowest CI most of the time.

AMS-ABZ is an interesting route - it's long enough that it's worth going up high, short enough that the aircraft will not be at max weight in most cases, and the high altitude winds are usually from the side so there's no point in trying to optimise for least headwind or most tailwind. So it seems reasonable to climb immediately to max cruise alt and stay there as long as you can - ATC permitting.