I have to side with the people who find different scopes to be ... different.
Let's assume that all these scopes have no edge distortion, brightness was for the most part the same, and color rendition was the same, just how the optical definition-often called "sharpness" looked through each.
Well, therein lies the trouble. To my knowlege, there is no aspherical glass in ANY brand of riflescope on the market. This is a lens or lenses which when ground, are not a portion of a sphere either on the convex or concave sides. High end camera lenses, read serious big bucks, have aspherical lenses. This is what eliminates (mostly) the edge sharpness troubles associated with spherical lenses. The variations in lens quality and number of lenses will affect the quality of the image you see and no scopes are created equal.
Brightness is also a function of how well the lenses are designed, and how well they are made. Also, some light is lost for every lens the light passes through. It's just life.
All these points affect the color you see. Think about it like this, if you looked at a rainbow and it was way out of focus, what would you really see? ... Nothing. all the light wouldn't be seperated anymore and you'd see just what you normaly see on a rainy day. Rain. As the light passes through the scope, the more "abberation" which takes place, that's, the more the immage gets "smeared around" in simple terms, the more the color is lost. This is why different camera lenses will reproduce nicer color than others on film. They transmit a more natural immage than a cheapo zoom and so it ends up not only brighter, it can be shot faster, there is better color representation, better saturation (color level) and the image is sharper.
Edge sharpness isn't such a big item in a riflescope. It's really overrated. We pretty much look down the centers. Do some looking sometime and REALLY look at the edge image in a riflescope. I think you'll find they pretty much suck for that since they are not designed to be a spotter. Look through a $150 spotting scope and then look through a Geovid or a Kowa (forgive me if I mispelled K.) Wow, what a difference edge to edge. But, that's what you pay for.
This discussion can go on and on but really, look in your scope and if you're happy, let it at that. You may just get 2 of the same model scope that aren't the same at certain distances. I have that myself now.
Another trick some mfgrs use is they reduce the field of view and that sharpens the edges so people think it's more clear. Well, no, they just don't show the outer edges of the immage. NF for one does that. It's like boresighting a gun. Where's the rest of the immage. It's bad enough you can't see through the reticle.
For the person who doesn't look through glass much to make a comparison. I would say that the best way to really compare scope to scope and have the result be fair is to look at color alone. Color is really a culmination of all the other quality factors combined. No light, no color. Distorted image, no color. Edges blurry, no color. Well, look for how pretty colors are through the scope and that's gonna tell 80% of the story. If the color is soso on a good day, then you positively can't use that scope at dusk or early morning for hunting cause it's gonna be d a r k.