So we're getting the ARG in June, RotRL in July and UE in August. NPC guide is coming out sometime in Q4. Hardback releases seem heavily weighted to the summer. Part of this may be Paizo metrics showing that this sells the most books. If there are going to be quarters without hardback release, it old be nice if we could get a crunchy/significant soft bound release during them.
It's more that the ARG's scheduled release got pushed back due to delays, as it was supposed to be an April release IIRC. The second hardcover in the RPG line has always been a GenCon release, so it's a set in stone(or as close to stone as they can get) August release. The third hardcover is generally a late Q3 or Q4 release, depending on how it slides due to shifting deadlines. Please note, this is all in regards to the 3 RPG hardbacks Paizo puts out a year.
The RotRL hardcover (and last year's Inner Sea Guide) were outside the normal product schedule, and end up falling when they can get them done. Both of those items saw delays from initial estimated release dates.