Yes, it's always been different between WDW and DL for
Genie+. DL always allowed pre-buying Genie+ for length of ticket, WDW did not. The new WDW system - LLMP - does allow for pre-booking, as does DL. At DL you can buy day by day, but had the option to purchase in advance - the old system at WDW did not allow pre-buying at all and was day by day only.
The new DL LLMP (
Lightning Lane Multi Pass) system is the exact same system as Genie+, only the name changed. The system itself still functions the same as Genie+ worked at DL. There's no reason for them to change the system at DL because overall it works very well.
The reason WDW changed the name and the functionality is because the system didn't work well there. This are several reasons, but a couple of primary ones are:
1) There are overall far more rides per park at DL than any individual WDW park (except Magic Kingdom), and many more people have hoppers at DL. This leads to overall better LL availability throughout the day at DL.
2) The demographics of people visiting DL and WDW are very different. DL caters to more locals, many not purchasing LL because of frequent visitation, so less people are using it. WDW has more out of town visitors willing to pay for LL because they visit less often.
The combination of these two things, plus not being able to book at DL until entering the park, make the former Genie+, now LLMP system, work better at DL.
Again, nothing at DL has changed about the system except the name - the functionality is the same as Genie+.