glock-hoser

Good afternoon! Does anybody know why the mag length for Open is different between USPSA and IPSC? For IPSC it's 170 mm and for USPSA it's 171.25 mm (though still being called 170 mm). I assume most shooters use 171.25 mm mags, except for when travelling to international competitions, but how is this done for the US IPSC Nationals, which I guess is run under IPSC rules? Is there a rule exeption made so everyone can compete with their 171.25 mm mags?