Understanding the eye prescription.

For my both parent in laws (60yr and 65 yr), I want to know whether the add power in bifocal lense has something to do with the prescribed eye report,or they put it considering the age. I'm curious because in their eye report there are values for terms like - distant, Near, cylinder and axis and the values are different for left and right eye. But in their newly made spectacles there is mentioned a new term - "add" and it is same for both the lenses. I got to know its the power of the lower lense. And they are complaining poor visibility in the lower lense. I want to know whether the "add" power is derived from the prescription or it's just an approximation considering their age?Should it be same or different for both eyes? Please help!