When you check the voltage of a 12 Volt golf cart battery a 12 volt reading does not mean it has a full charge. That shows the battery is very low and should be charged as soon as possible.
Having an even reading with each cell of a battery is important to how well it will preform. For both voltage and specific gravity readings, consistency among cells and among batteries is very important.
The voltage on a fully charged 12 volt golf cart battery in good condition will read 2.12 to 2.15 volts per cell (about 12.75 volts for a 12 volt battery, or 6.40 for a 6 volt)
Specific Gravity Readings to determine State of Charge: Use Hydrometer
12.75 & Above = 100% Charged
12.60 to 12.74 = 85 to 100% Charged
12.40 to 12.59 = 75 to 85% Charged
12.20 to 12.39 = 50 to 75% Charged
12.00 to 12.19 = 25 to 50% Charged
12.00 & Below = Fully Discharged
For more on Gravity Readings check here Why To Check Golf Cart Battery Specific Gravity With Hydrometer