The issue is almost certainly that you are flying at different times each day, and some of the return legs start on the same day, while some start after midnight and therefore on the following day. This means that while the outgoing leg has 1 flight per day, the return leg will have 1 flight on some days, but 2 or 0 flights on others. The days with 2 return flights are causing the oversupply warning. e.g. if the tuesday and thursday flights return after midnight, and the others on the same day, then outgoing will be 1234567, return will be 1-33
67, and you'll have an oversupply on wednesday & friday, no flights at all on Tuesday & Thursday.
Also, I really should save this response for future questions, save typing it again. It is a somewhat common issue.