It also has to do with your leverage point. Google pivot point and leverage mechanics of body movements ... Also, depending on the exercise, I am much stronger on tricep moves so it is definitely not that the bicep is stronger as much as the leverage comes into play.
For instance, my tri-push down with strict form max was around 175x8, my heaviest bb curl was 135x5 and that was in my 20s when I was experimenting with injects...the tripush was in my 30s all natural.