I have made a cursory search of the grey matter and I brought up some long time (and somewhat vague) memories on ATPL groundschool
on transition levels/altitudes. Are they not based on an altitude that clears all obstacles within the prescribed area by a minimum amount(formula used, min clearance increased with height of terrain), and are therefore terrain dependent?
When the formula is applied, it gives the maximum number of available flight levels as governed by terrain limitations.
On the other hand, it's been so long I might be mixing it up with something else.