I hope that's not a stupid question, but why define the syntax as 1/2 and 3/4 (divider) and not 1-2 (column 1 _to_ column 2) and 3-4 or 1->2 or something like that?
That was just an objectively bad syntax choice, but probably keeping on track with CSS.
On a side note: a problem with CSS' claim to be a programming language is that the grammar is not obvious.
Because of how spaces are already used to separate values in CSS (not to mention our own regular intuition when dashes are involved), this:
grid-column: 4 somegridarea / 6;
grid-column: span 3 / 6;
grid-column: span somegridarea / span someothergridarea;
grid-column: 4 somegridarea-6;
grid-column: span 3-6;
grid-column: span 3->6;
grid-column: span somegridarea-span someothergridarea;
grid-column: span somegridarea->span someothergridarea;