Why will some chainrings fit 7s-12s while others are meant for just 10s?

Question

When looking for a chainring, I see that Shimano specifies whether it is for 2x or 3x. I'm guessing that the pins (protruding rivets) on the former will be on one side only, and on both sides on the latter.

They're also labeled "PCD", but the numbers suggest that it is an alternative way for saying BCD.

Then they specify whether it is meant for 11s, 10s, 9s, 8s, or 8s/7s. I'm guessing that the thickness of the chainring exactly matches the exposed width of the pin between the inner plates. In other words, it would be possible to use, for example, a 10s chainring with a 9s chain, but the chain would wiggle slightly side-to-side. It would not be possible to use a 10s chainring with an 11s chain, because the chain would not sit properly. It would remain "elevated" at a larger radius (until eventually eating into the chainring to sit properly).

This all makes more or less sense. The puzzle is that some noname brands indicate "Machined Alloy Fits 7 to 12 Speed".

Why will some chainrings fit 7s-12s while others are meant for just, say, 10s?

Context of the question: I'm trying to drop my larger chainring from 50t to 48t or even 46t. I'm partial to faster rpms, and at 90rpm a 50t will let me approach 50kph, and that's simply not useful as I'm not racing.

Answer 1

Based on this article, you can see that chains from 5 to 8 speeds have the same inner width (2.38mm), and chains from 9 to 12 have the same inner width too (2.18mm). The difference is in the outer width.

In 1x (single chainrings) transmission, only the inside width matters, since the chainring is not meant to derail. The same chainring can be used for 9 to 12 speed systems, and also 5 to 8 since the spacing is wider, but you'll have a bit of play.

For 2x/3x systems, you have pins, but also guides in the chainring that are meant to help the chain to derail. These guides are sized to match the outer width of the chain. You can deviate a bit, that being said: I've often read that a "1-speed deviation" is perfectly acceptable. Personal example: I'm running a 10-speed chain on 9-speed chainring, it works perfectly.

And then you have another aspects, which labeling strategy: Shimano design their components as part of a system: the specs they write are "describing" the other components of the system rather than matching the actual capabilities of the product. Other manufacturers are less conservative and will try to have the widest specs as possible.