It's basically "the arbitrary padding the designer liked in the moment" vs. "the standard padding that's everywhere in the project." This book argues you should always use the standard padding. Your product should be pixel-perfect, just not in the PSD-to-HTML sense.

Oh yeah - I remember my first job things had to be pixel perfect to what the boss had mocked up in Photoshop. Thankfully at my next job it was just use whatever the project's CSS gives you unless it looks terrible.

I don't do much frontend any more but my current marketing team is happy with anything that looks reasonable.