I believe this is down to more than one positive rating being able to be given per post (ratings other than just simple likes), meaning that more positive reactions can be given than those that are counted as likes, as only explicit 'Likes' are counted towards the like counter. I'm not sure if this is the reason for this particular problem, but it's certainly a related issue.
There are still some improvements to be made here though to avoid confusion, but I'm going to mark this as 'Not a bug' for now.
