It really isn't. Plenty of ads end on Saturday without 11 starting on Sunday. I haven't noticed more ads ending on Sunday than Saturday. They always end on Saturday if 11 will start on Sunday just so they don't overlap. Even once in a great while the 11 ends on a Sunday so you get a day overlap of sale prices.
This has been debated on here plenty of times. It has changed over the years as the 11% would often overlap on Sunday when a regular sale would end and the 11% started, but I have not seen that happen in about 2 years.
Nearly always if a regular sale ad ends on a Saturday, 11% will follow the next week so that they do not overlap. The only time I have seen this not hold true in recent years are the several weeks surrounding black Friday where they do not want regular sale ads overlapping.
11% always ends on a Saturday whether it will be continued into the next week or not. Until somebody gets next weeks ad, we have no way of knowing what will follow an 11% week.
I have yet to see anybody prove these "hypotheses" wrong, but feel free to do so with an actual example.