It really just comes down to air space design.
If you have a corner parts of a sector that has tracks coming from 2 different FIRS, often there will be a change to an odd or even lvl to reduce the amount of separation involving 3 sectors in 3 different centers. You could imagine the co-ordination required in these situations, eg lvl changes, turns etc.
Or it maybe a busy route with a crossing route coming from another center with the same standard lvl, so to reduce workload for the planner controller the standard lvls are changed.
There a couple of examples I know of, these are going from Rhine FIR to Swiss, and Rhine to Malmo. You should always remember that in europe there are usually 2 controllers working one position, one planner with the other sectors, and one talking to captain awesome. So if the frequency is quiet, it some times does not mean the planner is not.