This is my thinking as well - There's SP3's from way back when still on display at various Microsoft stores (they don't replace the display copies if the incredibly dirty type-covers are any indication) - which don't have the yellow line, and there's also SP3's that do.
I think it's an either or situation, personally - but at the same time, my last SP3's faint line seemed to get stronger over time. Might've just been in my head, but either way - eventually this problem's going to get well-known and I didn't want to have to deal with even a faint yellow line impacting my ability to sell once the SP4 is released.
I've yet to see a screen made by anyone that's perfect across the board. The retina MacBook Pro is a good example. The people with Samsung screens complained about yellow tint and the people with LG screens complained about ghosting / burn in. I had the LG screen on mine and it was fine but people were going crazy exchanging them.
The people with Samsung screens were exchanging trying to get an LG screen and the people with LG screens were exchanging trying to get Samsung screens. It was pretty entertaining reading the Apple forum every day..
The SP3 that I had with the yellow line was very faint and I never even noticed it until someone posted about it here on the forum. To see my yellow line you had to view the SP3 screen at the right angle and in the proper lighting.
So... I returned the I7 - got a gaming laptop, the Razor Blade, still pretty small form factor for a laptop - and games ran great, but...
I missed my I7. Despite all the issues, I decided to return the Blade, and get an exchange for a new I7. No issues so far - MS store let me check for the yellow line that appears on the left side of the screen as reported in this forum, in-store. Went through two that had a fine yellow line, third one was the charm. Perfect white display now and happy to have the pen back in my hand. Can't wait for the SP4 with the atom chip and for all these minor issues to disappear with the next launch, hopefully.