If it was off enough to be visible, you'd have functional problems. Glock's success is plenty of evidence you don't need that much contact surface and/or tight clearance to have a functional, accurate gun.
As mentioned before, 90* with all square corners would be MUCH less durable than what Sig has done. Higher friction + stress concentrations = cracked metal.

I tried to google a better pic of the rails w/o a slide & came across an 80% p320 frame that the (de)structions say to 'hit the tabs with a hammer until they bend & fully contact the jig.
Precision Manufacturing
Maybe you need to use a glock as the hammer

Hiya!

These tabs are formed to conform to a specific datum point (actually a plane) on each tab underside, which together define the slide-frame bearing surface. It’s counter-intuitive (and wicked hard to describe in words), but the frame tabs need not be bent at 90 degrees for this datum point to be held within tolerance. I’m considered to be a smart man, and I sat in on a couple production engineering meetings where these tabs were discussed in minute detail. It took all that for me to sort of get it. You gotta trust me on this one.

BTW: examine your slide grooves in detail. They aren’t 90-degree box-way cuts.

After reading Bruce Gray's comment above, I pulled out a P320 Compact, and sure enough, the frame rail angle is clear, and as the slide sits mounted on the firearm, it's also very clear that it must be that angle to sit in the slide rail groove.

What surprised me about Mr. Gray's comment was that the bearing plane is the bottom of the rail, but it's clear that so long as the striker is cocked, upward pressure is exerted on the slide and it rides on the underside of the rail. With the striker released, the tension appears less, and I do note when removing the slide, wear on top of the rail, which I presume is the slide moving aft until the striker is cocked again (a very short slide travel).

As the trigger is pulled, the slide is pulled downward slightly, but generally rides with upward pressure along the underside of the rail.

I can also see that the cuts for the rail grooves are not square, and while the upper surface appears square across one side to the other on a plane tangent to the top of of the upper surface of the rails, the underside of the cuts appears angled down, a closer approximation to the rail angles. That's an unscientific eyeball observation and nothing more.

I'm not considered a smart man, and wouldn't likely gather the technicalities of the cut beyond what Bruce has explained.

What I do know is that it seems to work well, and it isn't a haphazard angle that simply catches the slide with a corner. Were that so, I'd expect substantially more wear on both the rails and slide. On this particular P320C, cerakoated brown (or tan), There's no significant wear on the slide finish inside, nor much on the rails either (yes, I use grease). The fit must be correct, else the parts would be wearing together, or one wearing the other, and presently the finish inside the slide and slide rails isn't harmed. This isn't a high round-count pistol by any means, but has probably seen a few thousand of my reloads, so if it didn't fit correctly and the angles match, there ought to be some indication.

quote:

Originally posted by Grayguns:
Hiya!

These tabs are formed to conform to a specific datum point (actually a plane) on each tab underside, which together define the slide-frame bearing surface. It’s counter-intuitive (and wicked hard to describe in words), but the frame tabs need not be bent at 90 degrees for this datum point to be held within tolerance. I’m considered to be a smart man, and I sat in on a couple production engineering meetings where these tabs were discussed in minute detail. It took all that for me to sort of get it. You gotta trust me on this one.

BTW: examine your slide grooves in detail. They aren’t 90-degree box-way cuts.

I have owned many Glocks and a few SIGs and this was the first time that I saw tabs bent in this manner, it didn't seem right that the bearing surfaces are points for all practical purposes and I was alarmed. It is obvious to me by now that this is by design and the design works and that's all that matters.
Glad I brought it up on this forum and learned that it's normal.
Thanks.