Which aftermarket sights are they? And are you sure that the sights installed on your refinished P226 slide were the originals? SIG could easily have replaced those with
new factory SIGlite sights. Below would be my explanation as to why they would do that.
The material used for most sight bodies is generally a pretty soft (steel) alloy mixture, intentionally so as to not cause any damage to the receiving slide when being installed. As such, in the installation process, the sight material is designed be the sacrificial material as the sight is pushed into place. Because of this the sight that you originally had on the P229 may no longer have provided a snug fit in the dovetail slot(s) after they tried reinstalling them on your refinished slide, though if that were the case it would've been appropriate to provide such an EXPLANATION rather than leaving it up for conjecture by the customer. But since you did have an aftermarket set on the P229, their tech probably figured that you would want the same kind of sight on the rebooted slide so he didn't put a set of SIG sights on...I guess...plus he actually wouldn't be doing a direct SIG product swap. Again, that conjecture thing.
At our shop over the years, I've had the (mis)fortune of reinstalling a variety of recycled, transferred pistol sights on various customer guns. In fact I just did one yesterday to a customer Gen5 G19 with a TruGlo tritium/FO set. Even though the rear sight utilizes a set screw and the TruGlo designs do go on relatively easily with less resistance than Ameriglo, Trijicon, Meprolight and other comparable sets, this TruGlo sight offered noticeably less frictional resistance than whet I've experienced in the past with new, right out-of-the-package sets. I suspected that these were recycled (the telltale was that they were brought into the shop in a Ziploc sandwich baggie), but the customer swore that they were "brand new". Hmm..maybe 'new' to him...
In years past I also did this with a couple of my own guns, namely a HK USPc and a Gen4 Glock 19. Most if not all of those recycled sights pushed back on far too easily, with little "fight" (resistance) like they commonly would've put up when they were brand new. I would always warn the customer that they went on far too easily, and that I could not guarantee that they would hold windage zero as a result of live fire or even dropping the slide during dry fire. I did as much with the Gen5 G19 customer from yesterday as well, not confident at all that the threadlocked set screw would be enough to keep that rear from drifting on him. But since he was a regular, I decided not to charge him for the installation. We'll likely find out how well it (doesn't) hold...
In several of the past cases the customer subsequently came back with a new set of sights to be installed, or bought a new set out of our own inventory, after their reused set started moving around on them. As for the rest who didn't come back, I'm probably guessing that they either didn't believe me, convinced that I intentionally 'sabotaged' their old sights in order to sell them a new set of sights, or just were prepping the gun with any set of sights to have as a backup pistol or some other reason and not really seriously using the firearm after the install. Or they really DID stay in place, and my cautionary warning was just that, a warning. But that's just conjecture as well.
With my own guns: for the Glock I wound up replacing the recycled Trijicon Bright & (not so) Tough three dots (Novak rear) after the 2nd or 3rd range visit because the rear sight started drifting on the G19 (they had been on my carry Gen4 G23 prior to the reuse). The Bright & Toughs on the USPc were totally out of whack elevation-wise (but I knew that going in when I first used them on another USPc; I just got used to how I had to hold the gun in order to hit where I wanted), but when they started shifting around sideways that was the final straw for them. The vials for both guns still had plenty of life left but as I was also moving away from 3-dot sights in general at the time, the inadequate fitting made it an opportune time to replace them with something more desirable.
Anyways, that's been my experience with reusing old sights. Others may have had far better results. Like I said...conjecture.
-MG