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Originally posted by 🍌IM A BANANA 🍌: they most likely have low casual mmr
This is exactly what's happening, thank you, Banana!
Quick Match (Casual) has an MMR system separate from Ranked that queues you with players within that 1,000 MMR limit. Since they have a high rank in Ranked, it seems like that's all they play, which wouldn't effect their Quick Match MMR.