The total resistance of a system is calculated by adding up the resistance of all the bands in the set and then multiplying by 2 to account for each end of the bands.
For example, if you take the 5lb. green band and anchor one end and stretch a single length of elastic you will be getting 5lbs. of resistance. If you anchor the band in the middle and have each end of the band attached to it's own handle, each end will be giving you 5lbs. of resistance. If you clip both ends to the same handle you will now be pulling 10lbs. of resistance.
With the 21pcs-262lbs system for example, the 8 bands add up to 131lbs.. That total then gets doubled to account for both sides of each elastic, bringing the total to 262lbs.