Filtered Voltage for charging Lead acid battery
When you measured the 14 volts I bet that was with the load and battery disconnected. Under load conditions, the transformer and bridge rectifier would drop a volt or two. 

Transformers are never perfect and the coils have series resistance that cause imperfect "regulation". Series leakage inductance also contributes to the problem.

For the rectifier, the volts lost across a single silicon diode might be about 0.5 volts on very light loads but when charging, each diode voltage will be more like 0.7 volts and, for a bridge rectifier lose an extra 0.4 volts in total.

You know the answer by now, add 5% (ish) more turns to the secondary.