Blending is more common than not. Most of the time it won't even be mentioned to you, it's just done. 2007 OEM Ford Black has about 9 legitimate tint variations. The variations should not exist, but runs on black cars are not done all at the same time, or in the same place. You don't just run 50,000 black cars off to cover the worlds needs, you do runs of lets say 50 cars black, 50 red, 50 yellow so on and so forth. Then process starts over and guess what, George who mixed paint on the last run was replace by Mike this time. Yes machines are used to get the mixes within spec, but chances are they don't mix the exact same because both people do the job in their own way.
The product changes too, cost will drive the use of another product and your nuts if you think Ford isn't thinning things out to make the overall cost even cheaper on their part. You think when they thin it's exactly the same every time? Not a chance.
As mentioned, tinting is simply a change in the mix. Subtle but it's a change, body shops and custom painters have to tint to get within spec of oem. As big cat mentioned every car is slightly off. Chances are it will never be noticed, but when you swap out an entire panel, lets say a side skirt or front facia, the seams are right up against each other and sometimes they eye will catch it. This is all that Stanged4life's painter is worried about.
None of this is rocket science, shops these days are beyond capable. Some folks just think they getting one pulled over on them. If thats the case, just let em shoot the piece, clear it and bolt it on. See if you notice anything. If not, your all good. If you see a subtle difference, you'll likely be headed back for a blend or tinted reshoot.