The solar cell is like a consent current generator. I hope I can explain this so it makes sense. watt=volts X amp. Lets say full sun and your panel is a 100 watts lets say it output is 20 volts no load that will be a max of 5 amp that the panel can deliver. with no load the panel is putting out 0 amp as you increase the load the current will increase until you hit 4 ohm at this point the current will not increase but the voltage will start dropping ( as you are now at the 100 watt that the panel can put out), so at 3 ohm load the current would still be 5 amp but now the voltage drops to 15 volt. You will now notice that the panel is only put out 75 watts. Volt = ohm X amp. If you take the 2 output wire of the panel and tie them together and read the amp you will still have 5 amp. If you measure the voltage at the panel you mite read 0.5 volts so the panel are only putting out 2.5 watts.
Now back to your 13.25 that is as high as your 8 amp can raise your battery voltage, as the battery charges the voltage will raise.
You say you had a current meter in the circuit what is it saying?