Essence Reaver doesn't give the base 10% cdr from the unique passive on the item.
Title. Was playing ezreal with essence reaver and triforce. The items physically stated I had 40% cdr but i only had 30% on my champion. I checked if it was just a visual bug and i actually had 40% but when i got blue buff my cdr went up and my abilities had a lower cooldown (on the ability text).