Game Director Ion Hazzikostas explained why the medium population realms still have queues; the algorithm pulls rankings dynamically, relative to other realms--but when Full realms have extreme queues, that means realms with better queues will be marked as Medium.
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Source: Blizzard Explains Why Medium Population Classic Servers Have Login Queues (https://www.wowhead.com/news=294750/blizzard-explains-why-medium-population-classic-servers-have-login-queues)