There appear to be some surface level differences in the trackpad coatings from batch to batch which might affect how they feel.
I have two MBA 15"s in front of me, both in midnight color, purchased a couple of months apart. The trackpads don't feel the same so I took a USB microscope to the surfaces - and to my surprise, the coatings look different.
The older one has a stickier and sluggish feel. The surface cells are larger.
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The second one has a smoother and more polished feel. The surface cells are smaller, roughly half the size. These images were taken at exactly the same microscope settings and zoom level.
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The midnight coatings on the other parts look very similar, and I don't notice any difference in fingerprints on the outer case, so it's just the trackpad that seems to be different.