It doesn't happen 100% of the time, only when your new pickup time/rental terms no longer qualify for the rate you had originally reserved, at that point the rental software will query the rate engine for the best rate available for the new rental terms and apply it. Heck, sometimes it even works in your favor and you get a lower rate! It's part of the risk management measures to prevent fraud programmed into most day/time dependent transaction software. Without those measures someone arriving at 6am on Thursday AM could make their reservation for noon to get the weekend rate quoted on their reservation even though their rental terms don't qualify, if the computer wasn't programmed to "catch" such irregularities there would be no integrity to pricing models, it'd pretty much be a free-for-all.

To give an example of how this can work in your favor at times, if the Thursday AM situation I described above was reversed and a customer reserved for a 6am pickup didn't arrive until noon or later the rate engine would return the lower weekend rate when the rental software runs its query.

To give an example of a situation where no change would be made, if I reserved a vehicle for a pickup at noon on Monday with a return at noon on the following Sunday and received a standard weekly rate (not a special promotion), but my flight was delayed and I didn't arrive to pick up my car until 3pm on Monday nothing would change because I was still well within the parameters of standard weekly pricing (minimum 5-day keep).

The world of revenue/yield management is a dark, hazy cloud of seemingly unrelated and non-sensical algorithms, it's not as simple as matching supply X to demand Y anymore (if it ever was!), but hopefully I've been able to shed some light instead of making things more confusing!