If the kite stores are using sail materials and quoting the manufacturers' data they might be using sailmaker's yards.
To convert weight from sailmaker's yards to square yards, multiply by 1.26.
From the glossary at Sailmaker.com:
An English/American unit of area for measuring the weight of sailcloth. It is 28.5 inches wide by 36 inches long. To say a piece of cloth is 6.5 ounce means a piece measuring 28.5" by 36" would weigh 6.5 ounces. This rather strange "yard" seems to be a remnant from the days when it was universally agreed that no one would ever be able to build a loom wider than 28.5 inches which would weave sail cloth. (Today 54 inches is common.)
Can we please convert to the metric system? I'm getting sick of ounces, pounds, fluid ounces, chains, rods, furlongs, miles, feet, yards, sailmaker's yards, etc.