Of possible interest, some explanation by Apple contained in the transcript of a presentation at the WWDC in 2015
https://developer.apple.com/videos/play/wwdc2015/804/?time=1688
"But I want to tell you about the spacing of numbers, their widths. By default, on our platforms, numbers are always monospaced, meaning they all have the same width, which is great if you need typeset something like tables, for example. Monospaced numbers are really handy because they make everything align. You can also achieve layouts with, like, multiple columns with them.
However, they are not the only numbers in the San Francisco fonts. We also have what are called proportional numbers, where each number has its own "natural" width. So there are situations where, of course, monospaced numbers are clearly what you want.
For example, like this, when you have proportional numbers, they will actually wiggle around, and monospace numbers actually stay put.
But you may also typeset numbers in static labels, such as when typesetting a date or a phone number or a read count in an inbox or an email address or a URL.
In those cases, really, you want proportional numbers instead. So let's take a -- do a case study with an app like Calendar, which is fairly numbers heavy.
And if you look at how we use numbers in this app and how we typeset them, you will notice that they don't really align with anything else, and they really are in line with other text, meaning they could use -- they should use, and they do use proportional numbers.
So knowing this, we decided to take a big leap here, and change the default of our platforms to actually give you proportional numbers by default and make you opt into monospaced numbers. Now, there is a precedent for this, and that's the Watch, which actually ships with proportional numbers by default.
But it is a big change, and it may affect layout. So because of that, we have provisions in place. If your app is not linked against iOS 10.11 -- sorry, OS X 10.11 and iOS 9, you still get monospaced numbers by default. The system turns on the monospace feature for you."
