I think the charts are conservative. I have often used wider tires than they recommend without any problems.

There's a trend these days to use wider rims with narrow tires, particularly 700x23 tires with rims that have a 23mm outside diameter. Typically, those charts would say that's a bad idea, but it's becoming a highly recommended configuration. I can only speculate as to how well that would translate to your rims and preferred tires.

Have you verified the 21mm inner width measurement yourself?