Watch Face battery percentage displays only 100, 80, 60, 40, 20 or 0%

My Watch Face Really Simple is showing the current battery percentage.

On my Forerunner 45 it is only displaying 100, 80, 60, 40, 20 or 0%.

It never shows a number in between.

Is this normal? Is there a way to show more detailed battery percentage?

Here is the code for the battery percentage:

hidden function setBatteryDisplay() {
var batteryDisplay = View.findDrawableById("BatteryDisplay");
var dataString = (System.getSystemStats().battery + 0.5).toNumber().toString() + "%";
batteryDisplay.setText(dataString);
batteryDisplay.setColor(getApp().getProperty("ForegroundColor") as Number); 
}