The reason is probably so they can show us a higher number. After all, if it ticked every second then 850 regen as listed now would have to be changed to 425. Players like larger numbers.
Also, the game design was setup with significantly lower numbers for our stats. A fraction of what it is now. When they added CP they upped our numbers 10 fold iirc. It was so a small change to CP would show up on our character sheet. So 850 regen now was probably more like 85 regen and try to divide that by 2, it becomes 42 regen and we lose one point (Zos rounds down in many things).