why do grid-column-start and grid-column-end both have spans in col-span-*?

[RungeZhai]

Class col-span-2 corresponds to style

grid-column: span 2 / span 2;

I think it should be

grid-column-end: span 2;

or

grid-column: auto / span 2;

Because at least one of the grid-column-start and grid-column-end has to be an absolute value, e.g. a grid line. If they are both number of tracks (relative values) how does the browser know where to place the item?

[RungeZhai]

Sorry, just saw the answer here. Apologies for not checking the existing discussion.