If your end volumes are on target, it's pretty much impossible to be off from the OG of an extract recipe. Gravity is a measurement of the amount of sugar dissolved in the water. Extract only has a certain amount of sugar in it, and you only have a certain amount of water, so there's really not any factors that can change in that regard (unlike all grain where the amount of sugar you get out of an amount of grain can vary, sometimes wildly).
So my guess is that one of two things is happening:
- You are ending up with a smaller volume than usual, either due to not adding enough top-up water and/or boiling off more water than you usually do.
-If you are adding top-up water, you just simply didn't mix it up enough and managed to take a sample from a spot in the wort that had a higher concentration of sugar.