macOS Github actions queued, "The agent pool assigned to this job has hit their MacOs concurrency limits", why?

Our team has (essentially) a CI system built in github actions, which spawns 5 testing jobs for a PR. This requires macOS github-hosted action runners. When more than one PR is being built, our jobs will often end up queued, with the following message:

The agent pool assigned to this job has hit their MacOs concurrency limits

Because github-hosted action runners are on-demand, this limit is surprising. What is it, and can it be fixed?

As of this writing (27 Apr 2023), the number of concurrent jobs running on github-hosted action runners is limited, based on your organization's GitHub plan, and runner type, like so:

And as usual on GitHub, anything related to macOS gets some sort of special penalty. As "Not applicable" might indicate -- and confirmed by my testing -- the concurrent macOS limit does not bump to 500 when switched to the "larger" -xl version of the macOS runner. Same old limits apply.

Otherwise, apparently these numbers can be upped only for Enterprise.

Note: If required, customers on enterprise plans can request a higher limit for concurrent jobs. For more information, contact GitHub Enterprise Support or your sales representative.

This information comes from github.