ANDROID fbset 3840x2160 but wm size 1920x2160 why?

Hello
I change resolution. modify uboot/board/kandas/config/kvim3.h
and /common/arch/arm64/boot/dts/amlogic/kvim3.dts file.

so fbset show 3840x2160 … but wm size is still 1920x1080…
anyone know… why fbset and wm size is different?