The editor just shows you the element’s position relative to whatever anchor you pick. If you want the element to actually sit in Minecraft’s center, anchor it to the “Center of Screen” and then set its X value to 0 (so it sits exactly on that anchor). Right now it’s still offset because you haven’t zeroed the offset after changing the anchor. If the element’s size changes later, enable Sticky Anchors on it so the center stays aligned.
For reference, the Positioning guide explains how anchor offsets work and how to keep things centered: Positioning Elements