You didn't choose it from the 'Display Fields' list (it isn't there), so you must have typed it in manually.
The reason why track rating displays properly in the track info pane is because it is set as a special 'block' in:
preferences -> layout -> configure panel -> 'show track rating' checkbox.
If you just type <Rating> into the display field, it does the same thing - shows as a numerical value.
So what you're asking is, could 'show album rating' be added as a checkbox in the 'show track details' dialogue.
I'd +1 this request on the basis that many people may use album ratings instead of full-on track ratings.
It takes 30-45 minutes to rate 300 albums, but rating the tracks could take 5-10 hours.