And further to that, just grabbed the battery from the evo and it's just popped up to 91%. I'll see if it goes all the way up.
I did notice however, that while the sensation and evo batteries have the same mah and model number, they are not the same.
the evo is 3.8v and 6.57Whr
The sensation is 3.7v and 6.40Whr.
The XL is 3.8v and 6.08Whr (smaller capacity)
Seems the XL might need the extra 0.1v to go all the way to 100%.
UPDATE: Yup, the evo battery charged all the way to 100%, so it looks like the 89% thing could be down to the voltage maybe....