If you use the decorate code 'A_ChangeFlag("FRIENDLY",1)', this will reduce the total monster count by one. However, if you use the ZScript code 'bFriendly = true' to do the same thing, then the total monster count will not be reduced. This will give you an extra monster that will not be accounted for if it died and you won't be able to get 100% kills.
Here is an example. The ZScript text contains TestMonster1 that uses 'bFriendly = true' after a few frames. The Decorate text uses TestMonster2 that uses 'A_ChangeFlag("FRIENDLY",1)' instead. Summon both to see the difference.