You have a very keen eye!
I do use slightly different thread thicknesses in different instances - there are certain variables that all change in tandem to meet an aesthetic that I'm after. I've got a few different ways that I stitch, and they have different advantages and disadvantages. One isn't 'stronger' or anything, but they lay differently, and allow for different thread thicknesses.
Sometimes it helps to have a thread that blends in and sometimes it's better to show subtle variation. Sometimes the back & front of the stitch needs to be a certain way, visually, and sometimes it doesn't.
All have some advantages and disadvantages, and usually craftspeople tend to settle into one or the another. I just find use in all of them, so I practice both. I see a few others do the same, but most only stitch one way and keep it that way (which I respect and admire all the same - there's beauty in the repetition of a solitary action over a lifetime, it's where the battles of mastery are won.)
I could go on and on about stitching - but perhaps it would keep me from the actual stitching
Love the input though! The difference in thread diameter rarely is more than .1mm from piece to piece - usually somewhat incomprehensible to most. It's usually just below the range of perception for most - not that it's impossible to see. Very interesting that you picked up on it.
I'd love to discuss thread diameters more... could you share the exact diameters of these two threads? Very curious!