I think both are most likely accurate depending on where your thermometer is placed. Maybe your heat tape is one temp and your air temp is another... that does seem like a wide gradient for an incubator to have though.

I usually adjust my thermostat up and down until the temps reach where I want them when using a temp gun. For example, I want my hot spot in my tubs to be about 88... So i'll continue to adjust the thermostat until my temp gun reads 88 when pointed at the bottom of the tub over the heat tape. This usually results in me having to set the thermostat to about 94 as the probe is directly on the tape vs in the tub.

in regards to incubating, I've always just incubated colubrid eggs at room temps in my snake room I put the incubation tubs on top of a shelf or on top of a stack of cages. My snake room temps vary quite a bit but usually they are around 78 during the day and 72 at night for most of the spring and summer then in the fall/winter it averages about 70-72 day and night.