What are your Charge Cycles?
The battery is expected to last 1000 Charge Cycles before it needs replacing at 80%, which works out to an average of 1% for every 50 Charge Cycles. As stated it is not a linear drop, just the average that is seen. You are probably around 200 Charge Cycles and anything more than that is better than average.
The one thing to remember is that the calibration of the battery occurs when the charge level is allowed to go to 100%. When using a Charge Limit, the phone will still charge to 100% on occasion to perform this calibration, as long as it has not been removed from the charger. Some users will prevent the phone from charging to 100% and that just prolongs the calibration, where they will see the larger than expected drop when it does get the chance to fully charge.