The cause of this is that the start of the spells start on the turn the enchanted unit begins. While this is not normally an issue when enchanting friendly units that can act during this time, it does become an issue when spells expire as soon as the enemy starts it's turn. One such obvious culprit is 'slow' which expires immediately upon the enemy beginning it's turn, and in effect only serves to lower the enemy's counter attack to 1, as apposed to functioning like it's description. All spells that have a duration that are cast on the enemy have their durations -1 because of this.