15g / 1.8mm spokes compatibility
The exact size of spoke holes in hubs is a little all over the place. There is some history involved, as well as dynamics with how manufacturers bias things for different most probable use cases.
Spokes with 1.8mm elbow areas used to be more common. The ubiquity of 2.0/1.8mm as the default butted spoke profile in particular is a fairly recent thing in the history of bikes. The 1.8/1.6 profile used to be way bigger, and a case for it can still be made in many applications. Aerolites and CX-Rays with 1.8mm elbows are still fairly common within their niches.
Manufacturers hate doubling the number of things they need to produce or creating compatibility gotchas (or at least some do, or used to, or whatever), so hub manufacturers are incentivized to make their holes work with both. There are no hubs I can think of that are 1.8mm-specific, but there are a number of higher end road hubs that are sized down small enough to barely work with 2.0 and be ideal with a 1.8. The holes on these are in the 2.3-2.4mm range.
Building such a hub with 2.0s is not a big deal for a handbuilder. It makes the spoke feel a little tighter going in and requires good aggressive technique to get them conformed around the flange, but it's commonly done and not remarkable. For a wheel production center it's a different story though. Human and machine hub loading both work better with a little more generous sized hole. 2.6mm is kind of the baseline for most modern hubs, but a lot of the most common OEM type hubs have 2.7mm-2.8mm holes. Some makers probably feel caught in the middle between the technical versus commercial advantages of each.
Running a 1.8 in a hole more optimized for a 2.0 doesn't have critical disadvantages. In my experience the main deal with it is that it takes more work to get the groove as established as it's ever going to be during the build process, since the mismatch in diameter means the elbow is going to dig in more. A fair number of wheels are built this way without major disadvantages.