How did you chill the pre-boil gravity sample prior to measuring? There can be a lot of evaporation when cooling a small sample, so you need to do it in a sealed container to get an accurate measurement.
If you are sure about the gravity reading, what about the volume? How was it measured? You need to have a very accurate way of measuring volume if you intend to compare efficiencies batch to batch. If you don't, then you could easily see a 5% swing just from measurement error.
Kai is right, if you leave behind 1/2 gallon in a 5 gallon batch you'll only get 90% of the efficiency you got 'into the kettle', or 63% if you had 70% in.