Is it normal for new redwood deck boards to shrink unevenly?

Half a centimetre of drift between two neighbours while the rest sit even usally points to moisture content differences between boards, so your instinct about the bundles is probly right. Redwood often goes down with a fair bit of water still in it, expecially if it was stacked outside at the yard. Boards from one bundle can be noticeably wetter than boards from another, and as each one dries it shrinks across its width. If two boards next to each other started at different moisture levels, the gap between them opens unevenly compared to the rest of the run.

The other thing I'd check is wether the screws are actually gripping. Redwood is quite soft, and it's easy to overdrive stainless screws so the head sinks in and stops doing much. A board with weak fixings can wander as it moves with temprature and humidity, while its well-screwed neighbours stay put. Go along the worst gaps and try gently levering a board edge with a flat bar. If it lifts away from the joist, the fixings there have let go. If the boards are still solid, my honest advice is leave it til spring. Gaps that open up from drying tend to keep opening a bit, then settle. Prying things back togather now, when the wood still has shrinking to do, just means you're fighting it. Come spring, if a gap still bothers you, you can pull a drifted board back against its neighbour with a couple of extra screws through the existing holes or slightly offset, which is a fifteen minute job on one board rather than ripping up a run. One thing worth doing now: mark the spots you're unhappy about with chalk and take a photo, so you've got a record of whether it's moving over winter or just sitting where it is. That tells you a lot about whether it's drying movement, which stops, or something structural, which doesn't. For what it's worth, that routine of a couple hours out there with the saw is a good one to hold onto, even after the last board goes down. There's always a fascia to fix or a handrail to sand.

A cheap pin moisture meter sorted this out for me years back when my cedar deck did the same thing. Turned out half my boards were still soggy from the yard and the other half had been drying in my garage for weeks. Now I let everything acclimate stacked under cover for a week or two before a single screw goes in. Since you've still got boards to lay for the stairs and fascia, might be worth letting those sit a bit first. And I hear you on the hours out there being the thing that holds you together. Mine got me thru a bad winter once too.

I noticed somethig similar on a garden bench I built last summer. The boards on the sunny side opened up way more than the ones that stayed shaded, so it wasn't just the wood, it was where it sat. Are the wide gaps on your deck all on one side, or scattered all over? And are both boards in a pair drifting apart evenly, or does it look like one shrank and the other barely moved? Might help you figure out if it's the wood or the sun doing it.