It probably isn't so much the rail, as the tolerances on the rear bridge of the Remington 700's Unfortunately they don't take as much care as they should when they finish / polish them. I've seen height differences of +- .015" from nominal, and worse, up to .012" of slope front to back across the rear bridge. It's also common for the rear bridge not to be even left to right. That is why I generally recommend epoxy bedding our bases, to ensure a straight, stress free mounting. It won't hurt anything if the tolerances are good and the mating surfaces have good contact, but will stop you from warping your base or action if it's a bad fit.
On a point another poster brought up, the price difference between some of the makes of bases generally comes down to material. All of our bases are either stainless steel, or titanium. Since they are both much tougher and harder than aluminum, they can only be cut at about 10% the speed of aluminum, or maybe 40% of the speed you can machine carbon steel. In this, like everything, time is money.
- Cameron