I thought it was a 280/310 gms, yard/ meter ratio. Meter being about 10% longer than a yard therefore a 10% weight difference.
Disclaimer: I made this up in my head as an explanation. It is not based in fact. I'll ask someone who knows next week. Stay tuned.
I always thought this was a weight "range" that depended on humidity and the moisture content that the cloth was carrying, hence you could expect the weight to be in that range? (Given that they all seem to quote between 30g, i.e. one ounce, which is a variable percentage depending on the cloth weight).
I'd be grateful for any first-hand observations of Smith Woolens worsteds, particularly in comparison to Minnis or Lesser 13oz. While I would have preferred a slightly heavier weight, I like the patterns, colors and feel of the swatches.