If people try this, they may wind up either not going at all or paying for the extra days. Unless something has changed since my last stay, Disney now does not allow you to simply add or subtract days from your stay. They will book a new-- shorter or longer-- reservation if those dates are available, but if the hotel is booked full for any of the dates within the range they will not allow you to drop the days. They changed the fine print to something along the lines of "reservation may be modified subject to availability." I remember a lot of people (including myself) had issues with this last winter. (I wasn't trying to subtract, I was trying to add dates that were available, but because some of the dates I already had booked were full they wouldn't allow me to modify.)
I think you're misreading what people mean by 60 + x days. Your original interpretation is correct. At 60 days out from check in date you can book for your entire trip. It's not 67 days out you can book because you get a bonus for staying for 7 days.
When people are referring to 60+3, +4, etc. they just mean that many days INTO your trip (not that many days BEFORE your trip). Ex. When you log on at your 60 days from check-in mark, you may not find anything for Flight of Passage until the third day of your trip (60+3)