I am using PIL to render text on images with position annotation. The text size changes after installing libraqm.

from PIL import Image, ImageFont, ImageDraw
font=ImageFont.truetype('Microsoft/msyhl.ttc',size=14,layout_engine=ImageFont.LAYOUT_RAQM)
print(font.getsize('a'))  
print(font.getsize('b'))  
print(font.getsize('c'))  
print(font.getsize('abc'))  

Result without libraqm

(7, 16)
(8, 16)
(7, 16)
(22, 16)

The sum of character's width 7+8+7=22 is correct.
Result after installing libraqm

(8, 16)
(9, 16)
(7, 16)
(23, 16)

The sum of character's width 8+9+7=24 doesn't equal to 23.
Also I find that the text size changes differently on different font after installing libraqm.
Why are these happening after installing libraqm?