All colonies at 100% approval, empire is 98% why?

I do not have any expansions or DLC. No Crusade, no Intrigue, no Retribution.

Like the title says, all my colonies are at 100% and have been for some time. But my empire's overall approval rating at the top right corner just stays on 98%, and has for a long time. I'm filling planet after planet up with morale buildings, and it's not budging. I have no idea why.

What's even more frustrating is that there's no information to tell me why this is. Mousing over approval score on a colony level lists all the bonuses and penalty to approval, but mousing over the overall empire's 98% approval doesn't reveal any information whatsoever, so I'm lost.

Any ideas please?