National - Different rates for slightly different pick up times?

I was looking to book a Thursday night->Sunday afternoon on an upcoming weekend and randomly ran into some interesting pricing. If I chose to pick up at 8PM the rate was lower than if I chose to pick up at 7:30 or 8:30. It wasn't a huge difference (a couple dollars a day) but it seemed really strange to me.

Does anyone have an explanation?

Is your dropoff time 7:30 for both? If so, I've had a similar issue:

http://www.flyertalk.com/forum/national/785952-weird-website-behavior-carlson-code.html

Nope. 3 PM for both.

I saw something similar on a recent weekend rental where a discount coupon was only applied on rentals of over 2 days

For my rental by extending the rental to 2 days and 1 hour a coupon became valid that was worth more than an extra hour.

Hence 2 days = x
2 days and 1 hour = x- $10
2 days and 2 hours ~ x

Something similar could be happening here where the specifics of the timing is determining whether a daily or weekend rate code is applied.

Just a guess.

Always play with the times when making a reservation. Often, I find that noon drop off and pick up is cheaper, especially if you make it on the half hour. If it's close to noon, then book it for noon.