In that case,it's most likely that you didn't get the chilled wort/top off watr mixed well enough to get a proper OG reading. Here's what I do; Pour both the wort & top off water through a fine mesh strainer in a roughly circular pattern. This not only filters out the big stuff,but causes the liquid to look like rain coming out the bottom of the strainer. This is how I get it aerated the most in this first step.
I then use my plastic paddle to stir briskly for 5 minutes straight. Mixes better & aerates even more. I get pretty accurate OG readings this way. Try it & see if it helps you. It does sound like it's coming out as it should,so this is the most likely culprit.