It's a bit unintuitive, but they key idea is roughly 'If you're working on stale load data (as always), best of 2 strikes the right balance between distributing load evenly and giving more work to less loaded hosts'. If you do 'best of k', you end up with herd behavior, overloading one host. 'best of 1' sends too much traffic to slow hosts.
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It turns out that best of 2 random draws outperforms best of 1, best of 3, and best of all in many load balancing scenarios: https://brooker.co.za/blog/2012/01/17/two-random.html
It's a bit unintuitive, but they key idea is roughly 'If you're working on stale load data (as always), best of 2 strikes the right balance between distributing load evenly and giving more work to less loaded hosts'. If you do 'best of k', you end up with herd behavior, overloading one host. 'best of 1' sends too much traffic to slow hosts.