Why the different reset times?

Daily activities are set to be for the day. They are "daily activities". So the time zone you signed up in is the midnight (plus or minus an hour for daylight savings) it will reset in.

Guild contributions reset based on the person who created the guild and their time zone.

Arena is meant to be a high activity time, so it's 6pm (give or take a daylight savings hour) based on the time zone you signed up in.

That is not always true. My spouse and I started the game on iOS at the same time. The phones are on he same account and set to the same time zone. Our reset times are an hour different, which is very annoying. We called the company and were told that people in the same time zone are not guaranteed the same reset/arena times and that there is no way to change it.

Hi @Sturmbock I think we're on the same server. :)
Have you checked out our server's forum thread?