More than likely you got full conversion, but had more beta-amalyse and limit-dextrines reactions, making for a more fermentable wort that will have less residual sweetness and body in the finished beer. This may lead to a lower FG than you had planned. As an example, I had a Belgian Golden (yes, that one
...) that I intended to mash at 60 C for 20min and 70 C for 40min. I missed low on the initial mash temp and the ramp up was slower than it should have been (caught it too late). The beer had an OG of 1.071 and a FG of 1.010. 85% attenuation when I was expecting 77%!