Your COBOL system normally uses a stack for handling PERFORM statements, while the RM/COBOL system associates a return address with a specific procedure name. As a result, under the RM/COBOL system all end-points of PERFORM statements are active until they are used, whereas under your COBOL system only the end-point of the innermost current PERFORM statement is active at any one time.
However, if the PERFORM-TYPE"RM" Compiler directive is set, your COBOL system will handle PERFORM statements in the way RM/COBOL does.