My incubator can differ in temperature by a whole degree Celsius and my hatches are still great. Rearranging the eggs daily helps to even everything out. I hand turn so this is easy to remember to do. Having your incubator full of eggs also stabilises the temperature so an empty incubator will read a different temperature to a full one. Remember to read the temperature at the level at which the yolk is sitting (this depends on if you incubate with the eggs upright or laying down).
My incubator gets wrapped in a blanket, leaving the vents at the top exposed, to prevent changes to the ambient air temperature having too much impact on the temperature inside the incubator.
Were your eggs shipped, or bought from someone local, or your own? Fertility will be reduced in winter and shipping can be very rough on eggs. I've had very low hatch rates from shipped eggs, though I tried another batch from the same place and those ones must've had a more gentle trip as I had a great hatch rate second time around.