I posted asking about this a while back, never got any meaningful responses. I go to New York almost every Labor Day weekend and this happens every year. And no, it's not a rate change during the stay, I see lots of "from $X" where the lowest rates during my intended stay are twice as much or more when I select the hotel. Haven't noticed it in other places. Can't explain it, and I have no solution other than checking lots of hotels to see what is really the cheapest for the area I want, and making a reservation & checking back weeks or months later to see if I can find a better one.