You should read #113 and #116
Now: using Aj's formula and converting your SG 1.064 to Brix i can tell you that the difference is not that big.
Your hydrometer : 111,6 gr/litre
Your Refractometer: 105 gr/litre
There is a 6 gr per litre difference or 11,1% strength vs 10,5% Less than 1%...