[g-b-r]

It's how they've been interpreted forever on Unix and other old systems

It's probably the main reason why they're so controversial.

[kbelder]

Which was influenced by tab stops on typewriters... which were adjustable, but were generally set at 1/2", 3/4", or 1"... which was the equivalent to 5-12 characters, depending on your font (monospaced, but sometimes 10 cpi, sometimes 12 cpi).