i think they assume people are under reporting their weight so they need to make up the difference
i recently did an online tux rental and the kept sending me sizes way too big and i finally had to call them and insist that they should give me the actual size i entered and that worked
if your measurements are correct there's no way you're a 34 and the 28 is probably just based on your overall height
but that's for standard builds, not everyone has same proportion, you seem to have longer than standard legs whereas i'm the opposite and a 32 at 6'2 where a lot of people my height are 34 or longer
you should ignore their recommendations imo unless it's a professional tailor doing specific measurements