Why are jobs being queued even though there are idle runners yet?

[dongho-jung]

I had 20 idle runners and ran 10 jobs.
I expected these 10 runners would run 10 jobs each immediately so that no jobs would being queued.
I took my expectation for granted because I thought the idle runner can run job immediately.
However they didn't work as my expectation. 5~7 jobs were being executed immediately though, 3~5 jobs couldn't get started immediately and then they started after 1~2 minutes.
How can I make them faster?

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p.s. queueing jobs said "Waiting for a runner to pick up this job..." even though there were idle runners can pick the jobs..