Always best to check all thermometers with boiling water to know where you stand with each. Check your oven thermo by first checking the probe one with boiling water. Then use your oven to compare the probe to the oven thermo by using both in the oven simultaneously. I am assuming here you mean a digital themometer with a remote probe
Your observation is atypical, but not unheard of. Is the grate thermometer near the edge of the grate? If so, it could be in the hotter air rising around the circumference of the water pan.
I have, in the past, had such an upside-down reading and attributed it to the digital probe reading cooler because of its proximity to the top vent on a breezy day. Also, be sure that probe isn't touching what's cooking on the top grate-- that will certainly affect the reading.