Unloaded batteries should measure 13.2 Volts as each cell when fully charged is 2.2V with 6 cells in total.
Adding a load will bring the volts down, most modern bikes have a minimal load due to modern electronics.
Alarms also have a current draw that loads the battery and will always drag down the voltage a small amount.
Often the colder the temp the lower the volts so bikes left outside over night or during the winter will also suffer a drop in voltage
Then i suppose the accuracy of any volt meter also needs to be considered.
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