Did you give it a vigorous stir before taking the gravity sample? It's probably due to non-uniform mixing. Fermentation is a violent, churning process... it'll mix it up just fine.
When I used to do extract with a top-up or split-boil I would take an electric mixer to it. Aerates and mixes well.
Primary: air and sadness =(
Kegged: air and sadness =(
Bottled: English Barleywine (brewed 9/26/09 -- bottled 5/5/10)
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