For a long time we specified displays by their vertical dimension -- 480p, 720p, 1080p.
Then the marketing guys came along and decided that the horizontal dimension sounds bigger. If we stuck with the less-bullshitty way of doing things and kept comparisons 1:1, we'd call 3840x2160 displays 2160p or "2K" displays, but instead, the marketing people decided that we're going to change things to horizontal and called 3840x2160 "4K".
Mixtral has an 8x7B model but it's actually 46.7B, not 56B params.
Kinda similar to how 4K displays are 3840 pixels wide, not true 4K which would be 4096. Marketing people called it 4K, not engineers.