We get this question 3-4 times every day, so you're not alone. And in reality, nothing's wrong.
It's a pretty common issue for ANYONE topping off with water in the fermenter (and that includes partial mashes, extract or all grain recipes) to have an error in reading the OG...In fact, it is actually nearly impossible to mix the wort and the top off water in a way to get an accurate OG reading...
Brewers get a low reading if they get more of the top off water than the wort, conversely they get a higher number if they grabbed more of the extract than the top off water in their sample.
RM-MN has a great analogy;
Originally Posted by RM-MN
This phenomenon is easy to see if you have a glass measure cup, some dark honey, and water. Pour in half a cup of water into the glass container, then dribble in some honey. Notice the honey sinks right to the bottom? It's more dense because of the sugar it contains. Now use a toothpick to stir the water above the honey. This will simulate using a spoon in a 5 gallon container. Did you get the honey mixed in? Not likely. If you drop in some dry yeast, it will find the honey and begin eating and the activity of the yeast will mix the honey in just fine.
When I am doing an extract with grain recipe I make sure to stir for a minimum of 5 minutes (whipping up a froth to aerate as well) before I draw a grav sample and pitch my yeast....It really is an effort to integrate the wort with the top off water...This is a fairly common new brewer issue we get on here...unless you under or over topped off or the final volume for the kit was 5 gallons and you topped off to 5.5, then the issue, sorry to say, is "operator error"
More than likely your true OG is really what it's supposed to be. And it will mix itself fine during fermentation.
And just use the number it says in the instructions as the true OG, because it will be.
So the answer is, relax and do nothing.