If Mozilla Studies are implemented the way other "push" update systems are, then probably your browser has an ID that it hashes to get a bucket ID that it builds into a URL to check for updates, plus a cron time offset for running those checks. Then, the experiments are rolled out by walking up the bucket ID list and gradually adding the addon to said buckets.
Usually, this mechanism is explained as being helpful to ensure a rollout of an experimental update can be rolled back if it's failing. That's not so much a concern in this case, I think. But this mechanism has another effect: it works as a solution to the thundering-herd problem. Every browser updating at once is bad, not just for Mozilla's servers, but for every piece of Internet infrastructure that those browsers (and their arbitrary set of addons) talk to when they update/restart. Within the time budget you have for running a rolling update, you ideally want as few machines updating concurrently as possible, just because you don't want to generate mysterious correlated traffic bursts that make NOCs paranoid.
Usually, this mechanism is explained as being helpful to ensure a rollout of an experimental update can be rolled back if it's failing. That's not so much a concern in this case, I think. But this mechanism has another effect: it works as a solution to the thundering-herd problem. Every browser updating at once is bad, not just for Mozilla's servers, but for every piece of Internet infrastructure that those browsers (and their arbitrary set of addons) talk to when they update/restart. Within the time budget you have for running a rolling update, you ideally want as few machines updating concurrently as possible, just because you don't want to generate mysterious correlated traffic bursts that make NOCs paranoid.