Splatoon 2 has dynamic resolution capped at 720p as far as I'm aware. It drops as low as 548p in HH mode to maintain performance.
The Switch HH is still stronger than WiiU and we've seen games WiiU ports perform better in HH mode compared to docked. So if Splatoon could run at a perfect 720p on WiiU but can't maintain that in HH something demanding is obviously going on. Not sure if they changed the ink physics or what.

It might also just be what others have already mentioned, that resolution optimization seems to be pretty low on nintendos laundry list and they are prioritizing other things in order to get the game out as soon as possible.

Even Zelda BotW that's actually buy far most demanding Wii U games is 900p.
Splatoon 2 is 2.5 months away from release, I think resolution at launch will be higher.

Yes its surprise if you see other Switch games from OP are running at 1080p and even Zelda BotW at 900p.

Probably to make sure it stays pegged at 60fps at all times.

The first Splatoon was far from perfect fps wise, huge drops on certain maps and situations.

I'd take 720p with perfect fps allday before 1080p with fps drops.

Zelda BotW ain't a fast multiplayer game. Different genres have different priorities.

But you can take your Switch to a friend's house just as easily as bringing an amiibo. 

I was surprised by Splatoon 2's resolution now that Digital Foundry basically confirmed it, especially as game after game is providing better resolution despite being far more graphically intense or (in the case of Snake Pass) having a lot more going on unseen due to the physics.

While it maintains 60 fps, I've seen enough from the other games analyzed by Digital Foundry to know that they're not being nearly ambitious enough with the resolution. Granted, the art style still allows for a beautiful game, but even just pushing it to 900p would be welcome and anything less is, really, pretty ridiculous.

Yes, and we already have Splatoon on Wii U running at 720p/60fps.

The game is 3 mos. away plenty of time to optimize both modes.

Yes, but Splatoon 2 <> Splatoon 1

Perhaps they want to use the performance difference for bigger levels, more details/effects... instead of just copying Splatoon 1 with higher resolution.