towards the "end" of the charge cycle, you might notice that going from (say) 95% to 100% charged, is much slower than going from 90-95% charged (or any other 5% charge increase.)
the PMU slows down the rate of charging to a "trickle charge" towards the end of the charge cycle.
it's designed to do this to avoid any overcharge issues. it's perfectly safe to leave the power cord attached all the time (although at least monthly you should run the battery through a complete discharge/recharge cycle to promote battery health/life)