You've taken a photo of the sightglass when holding the bike level on the tyres, now take another pic with it on the centerstand.
I'll bet there is near as dammit no difference in oil level.
The window on the C10 and C14 has an upper and lower mark that are both near the edges of the window so if you can see the oil level in the window, then it's probably within the acceptable range for level.
If you want to set your oil level to +/-1mm (39.37 Thousandths of an inch) then you either need a different hobby or need to see a shrink
I always do mine on the centerstand and have always done so on every bike that had a centerstand.
The only trick is to do it exactly the same way every time.
Put it on the centrestand with the engine running, and only then shut down the engine.
Walk away and come back 15 mins later to check the oil level.
I had to do the same with the bikes I've owned that used a dipstick (XS500, CB750F2, GS550E, etc.)
If you try to measure the oil level after just shutting down the engine, it will show an incorrect level regardless if the bike has a window or a dipstick.
If you move the bike in any way after shutting down the engine, then it may show an incorrect level.