Tilted knife back spine

When you look at any knife , the back spine is at 90 degrees from the sides of the blade. Right? Well, about two years ago, I bought a cheap folding knife ( about $ 6 ) and it was OK for me, but I noticed the spine of the knife was tilted to about 70 degrees. It didn't matter at all, and I thought ( what would you expect from a $6 knife ? Can't be perfect)

A few weeks ago, I was watching a few U-tubes and noticed a guy reviewing a knife and he mentioned specifically that this tilt in the spine was for scraping the ferro rod. Then I remembered an episode of Duall Survival when Copdy failed to start a fire and he asked Dave for his knife because the back of Cody's mora was rounded and Dave knife maybe squred ( for better friction with the ferro rod)

I went back to my "imperfect" knife and tested it with a ferro rod, and really it threw so much sparks I was afraid it would start fire ( I was sitting in my home office and expected the usual amount of sparks I get from other knives). The back of this knife works something like a Micro-chisel and really scrapes the ferro rod agressively. So, the rod may not last for long but you get better sparks.

A few days ago, I bought another cheap folder with two blades( I always wanted a folder with two blades) and noticed the same "imperfection" in thier backs.

Have you guys noticed this feature in your knives or is it not so wide-spread thing ?

I've purposely squared knife spines on belt sander, both at right angle and canted. If spine is truly sharp, it didn't matter what angle is it at, it'll scrape firesteel just as well.

I've only seen this in two knives I have, a Mora craftline allround which spine is factory grounded, and a SOG NW ranger that I can't tell since it's black coated. Never seen it in a folder.