Rob hit the nail on the head.
As you draw your bow the weight goes up in increments, usually 2 or 3 lbs per inch.
In a bow that stacks those poundage increments will will increase the further you get toward your anchor.
If it were on a line graph you would see it easily as that line showing the poundage at specific inches would start to climb at a very steep rate.
If you every pull one that stacks badly, you will understand the concept immediately.
Merry Christmas and God bless,Mudd