How does Measured (actual) AR differ from Mandrel AR?
Measured Aspect Ratio, also known as Actual AR, is the exact aspect ratio of a jump ring. It has been measured with calipers or micrometers. Mandrel Aspect Ratio, on the other hand, is calculated simply by using the mandrel size the jump ring was wrapped around as the Inner Diameter number.
These numbers are different because springback* causes the inner diameter of jump rings to be larger than the mandrel they are wrapped around.** For instance, Blue Buddha's D18 ALUM ring has a measured AR of 2.9 but its mandrel AR is 2.67 This is a variance of 8.7%
Generally speaking, one supplier’s Actual inner diameter of 1/8" (aka D, aka 3.2 mm) does not equal another suppliers. The same size may also differ across metals within a single supplier for many reasons, some of which include: temper of wire and size of the cut (kerf). However, the Mandrel AR for those 1/8" rings will be the same, since the Mandrel calculations do not take any of those variables into account.
*After wire is coiled it always "springs back" slightly from the mandrel once the tension on the coil is released. This makes the inner diameter of the jump ring larger than the mandrel itself, thereby making the Actual AR higher than the Mandrel AR
**Disclaimer: There is a rare exception to this - if your rings are very tiny and you use a thick saw to cut them, the resulting ring could have so much metal removed by the saw, proportionate to its size, that the ending inner diameter is actually smaller than the mandrel size. However, this also results in an oval-shaped ring that is not useful for most chainmaille weaves. Micromaille rings are often pinch-cut for this very reason - to avoid removing so much of the metal that the ring is no longer decently circular.
Why is the difference important?
Let's say you make a weave and you LOVE the way it looks and you want to duplicate it, only smaller. You measure your rings with calipers to determine the AR. In order to duplicate the weave, you would need to find rings of a smaller gauge with the same (or very close) actual AR.
If you don't have rings and you need to purchase them from a supplier, you should check to see if the supplier lists the actual ARs for their rings. If they do, you're in luck and hopefully can find what you need. If they do not list ARs, you will have to see which Mandrel AR gives you an AR that is closest, but on the slightly-smaller side - because remember the ring's actual AR will be bigger than the mandrel AR.