Because one day is not a season, and there is, in many cases, nothing for the second one to penalise.
I'm just wondering since that's still a season, just reflecting penalties for some interruption. 10 days off to 11 days off goes straight from all to nothing for these relatively simple tasks (sometimes states explicitly) while more complex tasks just take a -2 penalty?
Yes. They might be simple, but they take time. The same applies to all simple tasks, of course. The magus can do it in a season at a -20 penalty, so it goes from all to nothing at 20 to 21 days.
I'm actually a little surprised you didn't say the third (with the correction from ladyphoenix to 1 season - 20 days, not a single day) is the correct one. That's the one that fits the rules as written the best. I put all the others up as possible interpretations of what was intended instead of what was written.
Well, the third with the correction is not the version on the poll. The third option on the poll is definitely wrong.
Season - 20 or season - 10 are both reasonable interpretations. The essential point is that it should be impossible to do any other seasonal activities at the same time, or to do more than one of these activities in a single season.