Good question ... I have wondered that honestly. I do know back when woodies were used that even the same stick by the same company ended up with a different flex even if it said it was the same because it was wood which has natural differences stick to stick in the wood grain.
Composites ... well I do not know honestly company to company what is used to measure the flex. I have heard some guys i play with comment on one company being different than another, I personally have noticed that the blade lies differ company to company and would assume that the flex would as well but hey .... i'm no expert on the matter.
There's really no standard...I think at one point it was the amount of force required to flex the middle of the stick by 1", but the land of 70-75 flex is completely random. Some sticks are really stiff, some are whippy, just a crap shoot.
It's the force required to bend the stick one inch. Unless you're warrior and want to make people dependent on you because they don't know their flex outside of your retarded little numbering system. (Actually, that system began with harrow, but warrior kept it).
I think the inno made nike stuff was the first to use the 180 and up scale for flex. Then they kept it for the innovative branded sticks. I think only the pro stock warrior stuff is branded with non-traditional flex numbers.