Views pager jumps 2 months by 2

Hi
Using Calendar here http://labua.univ-angers.fr/agenda we've got a little problem : in the month view, the use of the pager (Prev-Bext) makes the calendar jumping two months by two where it should be more logical (of course) to jump month by month

Others views (Day, Week, Year) are working nice.
Any idea to fix that ?
Many Thanks
D