The OSTC sport computes the battery percent on the known consumption (Screen brightness, compass, etc). In order to work with bad batteries or not-full when inserted batteries there are several voltage-dependent thresholds for the Saft battery, all based on the worst case current consumption of about 70mA:
I'd assume the battery you used was either one of the counterfeit Saft batteries around, see [www.shearwater.com
] (We had these here many times in customer complaints already...), a battery that was not full or a contact issue between the battery cap and the housing. Also, an update to the latest 10.51 is recommended.