The charging circuit is low powered and is designed to keep a charged battery charged. If the battery is full and the engine is up over say a thousand rpms the voltage will be up around 14.5V. If you had a hard time starting the machine and were on the starter quite a bit and finally get it running, that voltage could be down around 13V or so. As you mow it will charge back up. If the charging circuit is not functioning the battery voltage will be less than 13V....a new, fully charged battery around here runs about 12.7V, so that is the highest voltage you could see.
If you have been mowing with things working properly and you shut down the machine and check the voltage immediately, it will be above 12.7 but letting it sit awhile the chemical solution will stabilize down to the actual charge voltage level.