Batteries in series

I assume you are making all the measurements in no-load condition. 

In that case, I can only think of two reasons, one your multimeter might have very poor accuracy or your battery is damaged. Also, make sure your terminals are not loose 

Agree with the comment above me. 

When connecting your batteries in Series you are doubling the voltage while maintaining the same capacity rating (amp hours). This might be used in a scooter, Power Wheels kids vehicle, or other applications. Just use a jumper wire between the negative of the first battery and the positive of the second battery.

Batteries connected in series stack their voltages in order to run machinery that requires higher voltage amounts. This can prove tricky when attempting to charge your battery, because it becomes necessary to account for the increased voltage levels. Because of the increased voltage, it is crucial to utilize a charger that satisfies the increased voltage output of a series configuration. Keep in mind that batteries connected in series have no effect on the amp-hour capacity of your configuration, thus, when charging a battery it is important to focus strictly on the increased voltage.