Strange movement of the Planning Time fence

We have a material that is set-up as follows

MRP type - P1

Lot Size - WB

I would expect that the planning time fence would move on Monday 00:00 (start of a new week), we ave seen the planning time fence move on a Tuesday and this week it moved on Wednesday.

Why is this happening? we are not using any Planning calendars, do I need to set somethin up.

thanks in advance