"A while back, we changed the default method of using Export As to working with the new UXP-based method. The older CEP-based method (which used to be the default) is now activated by enabling the "Use Legacy" option in Preferences.
One of the big differences between the two methods is the Quality setting for JPG files. In CEP, it used a API which allowed for a 1-100% quality scale, which is similar to the one used by Save For Web. UXP has an API which only allows for a scale of 1-7, which is similar to what is used in Save As. This scale is equivalent to approximately 15-20% jumps on the old scale. So 7 on the new scale is the same as 100% on the old, 6 is around 80%, and so on. Unfortunately, this is not something we can readily change, we are reliant on another piece of technology for this feature and are currently restrained by its limitations.
The 1-7 scale is a more recent UI change, from the old descriptors scale (Poor, Good, Great, etc). We only changed these labels to numbers, to make it easier to understand. We didn't change anything at all about the underlying JPG compression: 7 is exactly the same as Great and so on.
In your screen shots, we see exactly what we expect to see: 7 (from a scale of 1-7) in the new UXP settings ("Legacy" setting is off) is high quality. But 7 using (from a scale of 1-100) in the old CEP settings (Legacy is on) is low quality".