Compress images = A but 2 need compress and 5 need progressive?

[RobMcP]

I have not noticed this before, usually I will try to ensure images are compressed, and that they are Progressive.

Can anyone tell me how on a recent test, I manage an A score on images when 2 need compressing, and 5 are flagged as needing saved as progressive?

Compress images 90/100
Progressive 63/100
Yet tests with better scores than above get a D score. Confused...

Also (related) Images recommended for compression e.g.
34kb compressed to 12kb saving approx 20kb What criteria does Webpagetest.org use to decide that is a reasonable value, are there any guidelines or info relating to this?