Could be a couple of things.
Is the job set on an 12 weekly cleaning schedual?
Do you have your settings set so jobs can be cleaned on any day? For me my jobs are set for Mon through to Fri. So if a jobs next due date was to fall on a Sat it would move it to the following Monday.
Was the last time the job was cleaned done on the day it was due? If not, if the job was done a few days late you have the option in your settings for it to be scheduled 12 weeks from the date it was actually cleaned or 12 weeks from when it should have been cleaned, you may want to check how you have your jobs are scheduled if not cleaned exactly when there due.