Why in the store, Ultima 4+5+6 is stated release on 1985
While 1+2+3 is stated release on 1989

This baffles me completely, unless earlier games were released as a set after the later games as a retro package or such. And being the latest release that is what they used.

As far as I can tell, yup, that's the case. Ultimas I-III were re-released in a single compilation package in 1989, using the 1986 remake of Ultima I (which is also the version available here on GOG). IV-VI were also released as a compilation package in 1992, though I'm not sure if that's the version GOG worked off of for their collection here. I do know Ultima IV was released in 1985, which would explain that particular date, and considering Ultima IV is available separate from the rest of the trilogy, that might be why they're not using the compilation's date.

The first Ultima games were released for he Apple II and took a few years to be ported (or their remakes to be ported) to DOS.

So 1, 2 and 3 are older than 4 and 5 (6 was the first Ultima game specifiically targeted at the PC demographic), the version of those games that we get to buy here may technically be younger.

Also
Release dates

Ultima 4+5+6 - June 15, 1985
Ultima 4: Quest of the Avatar - September 16, 1985