My guess is that there are a number of things going on, all of which add up to your final efficiency being lower than you would expect.
First, there are the volumes. If your pre-boil volume is a bit lower than your estimate, that would account for something, and if the post boil volume is higher, that would account for a bit more. How do you measure your volumes? Using Ale Pales is notoriously inaccurate. If you have a good scale, weight could be more accurate.
Next, there is the temperature correction on the gravity readings. These are usually failrly accurate with temperatures below 80 - 90 degrees, but can be wildly inaccurate at sparge temperatures.
I'm guessing that you measured the 1.057 after transferring from the kettle to the fermenter. Was there anything left in the kettle after transferring to the fermenter? I have to leave a bit behind in the kettle, and the wort chiller. These will adversely affect efficiency.
Where did you get the sample to measure the OG? Was it taken from the 6g transferred to the fermenter, or was it in addition to the fermenter volume?
Then, did you take the OG reading after adding a starter? The starter will have a lower gravity than the wort, and will lower the OG when added.
Lastly, you will get some wort absorbed by the hops during the boil. This can be quite substantial if you use whole hops, and this will result in a decrease in efficiency.
Hope this helps.
-a