A unique visitor is a de-duplicated browser, not a verified person. The tool remembers a cookie, or a similar flag, and counts that flag once inside the range you asked about. Clear the cookie and the same person is new. Open the site on a phone and a laptop and one person is two uniques. Share a computer and two people are one.
Adding up daily uniques does not produce weekly uniques. Someone who returned each day is one unique for the week. Sum the days and you have counted them seven times.
MapMyVisitors does not publish this number. Its counter increments on each widget load. Calling that load a unique visitor is a different claim from the one the tool made. The longer explanation is what is a unique visitor. If a report you do trust says "uniques," ask it whether the identifier is a cookie and how long it lives before you compare two periods.
