I know ISO 8601, but that doesn't answer my question. Let me rephrase this, why was a format intuitive to a machine chosen instead of a format which feels more intuitive to a human?
I think YYYY/MM/DD - YYYY/MM/DD will mean the correct thing to many more people than YYYY-MM-DD / YYYY-MM-DD, which I suspect many will interpret as an OR and not a RANGE.
Personally I think it goes a little over-flexible on support for non-strict formatted dates to the point it becomes difficult to figure out what a given date string is going to result in rather than feeling easier to use. That's probably part of the reason the primary example uses the clearer format.
I think YYYY/MM/DD - YYYY/MM/DD will mean the correct thing to many more people than YYYY-MM-DD / YYYY-MM-DD, which I suspect many will interpret as an OR and not a RANGE.