Time differences between successive moonrises

Dear forum,

after printing the moonrise and -set data for my position in central
europe from the US Naval Observatory and calculating the time
differences between successive moonrises from one new moon to the
following new moon for March/April, June/July/, September/October and
December/January I found the following regularity : The time
differences are shortest when the moon is at the most southern
position of it´s orbit and are longest when the moon is at the most
northern position. Can somebody explain me the reason behind that ?
Could it be that those positions fall into the points closest and
farest to/from earth of the moons excentric orbit ? Any input is very
welcome !

Mr.Adams