Hi,
The observed trend is expected and correct because of the way Adobe Analytics unique visitors are calculated in different date period granularities. Within each defined date granularity (day, week, month) the unique visitors report will de-duplicate the count to a single visitor. Therefore at a day level you will observer a higher count over weekly UVs and monthly UVs.
Help Article Link on Unique VIsitor Reports:
http://microsite.omniture.com/t2/help/en_US/reference/reports_unique_visitors_v15_dsc.html
Best,
Brian