Why does ffmpeg convert from jpg to WebP make a difference in results?

JPEG typically performs irreversible compression.That is, it deteriorates when saving with -vf scale=1200:-1.

How do I write the results of pattern 2 in one sentence?

It's impossible.Pattern 1 encodes webp without degradation.

Also, if you save it in PNG format in pattern 2, it will be reversibly compressed, so the deterioration will be suppressed.However, the color changes because the color space is YCbCr in JPEG, while the PNG is RGB.