Calvinists generally face the issue of God punishing people for acts that they cannot avoid, since Calvinists deny libertarian free will and maintain we cannot do otherwise than what God has decreed for us to do, but supra-lapsarianism faces two additional issues: 1) God reprobates certain men for some reason other than their sins and 2) God necessitates the fall of mankind in order to accomplish election and reprobation. Supra-lapsarians believe that in the order of decrees ( which is a logical ordering of God’s plan from eternity, rather than a temporal order of the execution of His plan in time) election and reprobation come before the fall. In this sense the fall and sin are not the reasons God reprobates. So supra-lapsarians hold that God reprobates for some reason other than sin. After God has reprobated, He needs man to sin and be in a fallen condition, so He decrees the fall to accomplishing reprobation.
Infra-lapsarians avoid these two issues by saying that in the logical order, the decree of election comes after the fall.