Difference between lossless and lossy with jxl_distance=0

T Adobe DNG Converter command line interface documentation, it says `-jxl_distance 0` means lossless

I did my test on a 24MP RGB raw image, when using `-c -jxl_effort 9`, the output is ~80MB, takes ~2s, when using `-jxl_distance 0 -jxl_effort`, the outptut is ~50MB, takes 20s. 

Why is there a difference? Are they 100% equivalent?