roaster oven temperature is off

Taking a random stab here—and yes, this is a simplification of the physics involved—

If you measured the water temperature, this is expected, assuming you're talking about °F. Water will not get above boiling (212°F, at sea level) no matter how much heat you apply, unless you're using a pressure cooker. Every bit of heat you add once you get it to boiling just goes to boiling it off faster.

Now, as to why it isn't at 212°F, that's more complicated. The water is actually evaporating even below boiling; and this takes energy. (You've no doubt noticed a pot starts to steam before it boils, and that a lid on top will fog up too.) Energy is conserved, so that energy comes from somewhere—it turns out to lower the temperature of the water. How much water is evaporating depends on the humidity in the oven, which depends on the temperature of the air in the oven, how well-sealed it is,—i.e., is complicated. But at lower oven temperatures, it turns out that uncovered water will often be a fair bit below boiling.

The second possibility is that you are indeed measuring the air temperature, and you're seeing this, and there are a few things that could cause this:

• The roaster's thermostat could be wrong. I haven't used that appliance, but this is fully expected on normal ovens, even expensive ovens.
• The roaster may heat unevenly, and you're measuring in a different place than the thermostat.
• The roaster may not have the heating power and/or design to overcome the cooling effects from the water, described above. E.g., if all the heat is coming from below the water, the water may be limiting the maximum temperature of your roaster. A full-size oven, for example, could easily use 20 amps @ 240V when heating. The roaster probably plugs into a 15A @ 120V outlet, so its limited to 12A—less than ? the power.
• You may have measurement errors. E.g., if you're opening the lid to check the temperature, even a quick thermometer like a Thermopen would take 10–15s to get a reading (a Thermopen is 3s in liquid/meat/etc., not air, because those conduct heat much quicker). Obviously, this would let a lot of hot air out.