Yep....sometimes they'll change more than once in the same day!
Another thing I've noticed....I can be doing some price checking and put in a pick up time (or drop off time) that is an hour different and get a different price. Just can't figure out a reasoning on that...so the same car, at the same airport for the same number of days costs more because it pick it up an hour later....or earlier? And I'm not talking about an extra hour that makes it go over the 24 hours so it's really an extra day. I'm talking one reservation with a 9am pick up, 4 days, 7pm drop off. Next reservation, same 4 days, but a 10am pick up, 7pm drop off. $17 more. Ummm, hello, I'm getting LESS time with the car technically.
SO....play around with the pick up/drop off time......they'll hold a car for 3 hours after the pick up time, so if your flight arrives at 9am, try 7 or 8am and 10 or 11 am.