Maybe it's a trick of the wording ("branch circuits shall use minimum #12 THHN wire...") or something like that, which would imply 20A? I can't recite any particular code either, but I can ask a few journeyman electrician friends. Maybe @BillConnerASTC would know. I wouldn't be surprised if the OP's space is wired with #12.
It's possible the receptacles are actually rated as 20 amp edison, but at first glance you think 15. A look at the breaker would be informational.