Since we are dealing with vectors vs raster images a text object is only a "object"
A letter may require less or more “realestate” so the object dimensions need to account for that. So the first and last letter may not actually coincide with left/right object boundary.
This is not an Easel thing, all vector/design programs I have used act the same.
As Neil suggest, exploding the letters into individual objects will help as the extra room around the letters are removed during the explode operation.
I havent used Easel “center” function myself so I dont have much to share there unfortunately.