3.6 inches per second isn't that fast
It's rare that 2 players running full pelt against each other. The Burnley offside for example was a static defender's foot against a static but moving to lean attacker so minimal margin of error at such slow speed difference. Many more offsides are usually the defender holding a static line and the attacker leaning forward to start his run.
If you'd seen my old posts about this ages ago, I even suggested that they can interpolate the frames easily. They know where the players are over a number of frames so basically could just fill in the frame gaps with fluid motion. This is what modern TVs do with motion blurring elimination tech.
That's it for me as I've said this all before and it's not a science at the end of the day debating an inch here or there, it's football. It's got to be onside or offside somewhere but also consistent enough unlike the complete randomness that they got with linesmen in the past.
Given you get some and you don't then for me, being level with the white wall is fine as long as there's an advantage to the attacker, albeit I'd like it to be more but even now it's a bit to the attacker and a little bit to the defender. We may get a close one given offside next week that is down to an inch but then the next game, or even the same game it would be in our favour.