Leakage inductance and turns ratio
Just think about the primary on its own...

If each turn on the primary is ideally coupled to every other primary turn, one would expect inductance to quadruple for every doubling of turns. This doesn't quite happen - take one turn in the primary; the turns closest to it couple well with that turn but turns further away couple less well. This is because of the finite permeability of the core material. If the inductor were air-cored this would be obvious - turns further away hardly couple to each other at all.

So, having a core with significant permeability does help alleviate the problem but there are still local lines of flux around each turn that just do not enter the core and therefore do not couple with turns at some distance from them.

If you had a single turn secondary, it would couple more readily to the primary turns closest to it but those turns further away it would couple less effectively. The more "distributed" secondary turns you have the better are your chances of coupling with those far-away primary turns.