If it's an extract batch that you achieved the correct volume in your fermeter, then the OG of your beer is what the recipe or brewing software says it will be. Since you are not converting starch to sugar yourself, the gravity of your extract kit is what the extract was at the maltser, dilluted by the volume of water you used. It's already been calculated, and if you hit the correct volume, it is foolproof.
It's a pretty common issue for ANYONE topping off with water in the fermenter (and that includes partial mashes, extract or all grain revcipes) 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.
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.