This is normal. I'm guessing the mac's charger uses a constant voltage (CV) charging method towards the later(ending) charging stage. The charging current will logarithmically become less and less at the battery reaches the peak voltage. This is certified to be the safer way to top up a li-ion battery to it's maximum capacity without overcharging it.

The earlier stage of charging uses a constant current (CC) method to provide the maximum charging current at a fast constant rate which is why it charges much faster before it reaches the point where the charger switches to CV charging mode. That is I'm assuming that mac's does it this way as most modern chargers do.

Don't forget, the batteries don't get charged if the notebook is plugged in 24/7.
It's a myth that the battery will keep charging if plugged in. All consumer devices with lithium-ion batteries have proper protection and charging circuitry. Because any lithium-ion cobalt battery that is overcharged or overdischarged can turn into a bomb when it experiences thermal runaway.

The only enemy to the batteries are heat.