A 12V battery is actually closer to 13 volts. 12V is used as its nominal voltage. Ie. 12v for one battery in a circuit and 24 volts for 2 batteries in a circuit.

A 12 volt battery is actually 6 lots of 2 volt cells in series. Depending on the type of lead acid battery the individual cells can vary in voltages from 2.08 up to 2.16 volts each cell. Using that maths 2.16 x 6 means your 12 volt battery is actually 13 volts like an Optima Battery or at 12.84 volts an Odyssey, both these brands are AGM batteries and they have a higher voltage than wet cell batteries which might only be 12.5 volts in total.