Why do some USB-C docks claim 4K but don’t actually run 4K properly?

Is it bad marketing? Or is there something about USB-C bandwidth or port type that I’m not understanding?

I’m trying to get a clean dual monitor setup going (two 4K,) with my MacBook air m3, and I don’t want to drop money on a dock that says “4K” but ends up looking like a stretched-out YouTube thumbnail.

If you’ve got a dock that’s reliably giving you full 4K at 60Hz — especially on Mac — I’d love to know which one. Also curious if anyone’s dealt with these “fake 4K” docks and what actually causes the issue. Bad HDMI port version? Misleading specs?

Thanks in advance. My eyes just want sharp text.