Bay Lake is a sold out resort. Polynesian is still in direct sales since the Tower opened. They haven't even declared all the rooms at the Poly. All the non-sold out resorts will have the best "specials", but they will have an extra incentive on a sold out resort once in a while. It's currently AKL. It was SSR last time.
From what I've read about the 2 you are looking at and after watching many videos of them, point wise, BLT is a much better value. Comparing one bedroom units, they are both highly desirable and both offer 2 bathrooms, but the
point charts are significantly different.
I actually bought Poly for the overall vibe, but I will be very happy to stay at BLT and save some points once in a while.