I know there is only one official spec for a picatinny rail but I recently ran into an odd issue that has me wondering if some companies are using a non-spec rail for some reason (or, I guess, why they are, not if, since my issue clearly shows they are).

I have a first generation Vortex UH-1, it fits most rails I have tried it on, a Spike's AR upper, Scorpion, Bren 2, Kriss Vector.

It won't fit, however, on a couple of random things. An SP5 B&T claw mount and a Flux Raider, for instance.

There seems to be two issues with the rails it won't fit on.

1) The "tooth" on the UH-1 that fits over the rail and locks under it appears to be too long to fit over the non-spec rail

2) That same tooth is too thick to fit between the underside of the rail and the frame of the gun

At first I thought it might be an American/Euro thing but then I tried all the guns listed above and you can see there's some from both regions on either side of the issue.

I guess it could be an issue with the UH-1 and not the rails. The UH-1 is a beefy optic so maybe they needed a thicker mount then usual?

While they look the same, you may be dealing with a Picatinny v. Weaver rail issue
Brownells - Picatinny vs Weaver dimensions

Sometimes the machining is out of spec.  I had an upper that a BUIS would not fit on.  It fit on all of my other ones.

I don't think that's the issue as the main difference between pic and weaver is the grooves, IIRC.

This seems to be more of an issue with the amount of space some companies leave (or don't) between the underside of the rail and frame.

I'll try and get some caliper measurements at some point and see if I can pinpoint the exact dimensional differences causing the issue.

Absolutely. I got some off amazon just so I could use my prime now an get them fast. So loose the light just rattles back and forth while bcm ones I have were super tight.

Yup, I have several ARs & non-ARs with ‘picatinney’ rails that vary.
Usually not an issue with most optics/accessories but I have a UTG pro mount that has square tabs where the side screws are that fit a mil spec rail like a glove but I’ve had to file other rails. Not sure if they were Weaver spec rails.

I just mounted a 3bucc brass catcher and the mount is one piece that slips on the end of the rail. Fits all my uppers fine except for my Alpha Shooting Sport upper. The lip/flare on the sides of the receiver rail is a slightly different profile from the rest but has never been an issue with any other accessory including the UTG pro riser.

Interesting topic. Yes I did have some issues here and there with stuff not fitting on the rails. Tolerances I guess. Never thought much about it ...

Technically speaking, yes.

There are those which conform to STANAG 2324/MIL-STD-1913 rail specifications (true picatinny) and then the more recently adopted STANAG 4694 NATO accessory rail specifications which was developed to offer greater mounting repeatability.

Though from a pure, what can or cant attach aspect their differences are negligible.

Additionally flashlight rails on handguns (and subsequently many flash lights) do deviate from both of the above specs quite often due to form factor and manufacturing technology used in production. Molding a to-the-spec pic rail on a frame is a much larger hassle than it's generally worth (costly post op machining is about the only realistic way to be completely accurate) when the bulk of consumers won't attach anything there at all and those that do are by a large margin only attaching flash lights. By that same token, you can pretty much assume any polymer picatinny rail such as on a cz scorpion or various keltec product will also be more along the lines of a generally compatible approximation rather than true to specification mounting system. Though the basic design premise is quite simple and forgiving so these approximations very rarely run into compatibility issues.

Lastly you have things like the weaver rail which is very similar from a front profile, but deviates in groove pattern spacing and tolerancing seems to be at the manufacturer's discretion.

Maybe not directly related to the topic and I have no personal experience with the following. Airsoft rails are 20mm, mil std 1913 rails are 21.2mm. There are a surprising number of big name BUIS/fixed sight manufacturers who license copies of their products to the airsoft market. I'm sure other firearm rail mounted accessories are the same. These products can look identical to the real thing except for the mount dimension. Apologies for not helping with OPs problem.