Thermal sensitivity and thermal accuracy are different specifications. There is another factor that you need to be aware of which is spot size. An infrared imager (imaging radiometer) cannot accurately measure temperatures at the single sensor (pixel level)
Spot size defines the minimum size object that can be accurately measured with a given lens at a particular distance with a particular imager. Anything smaller than the spot size cannot be accurately measured, although it may be readily seen. The spot size will cover some number of sensors and is an important consideration when performing quantitative anaysis.
Thermal sensitivity specifies the minimum change in signal due to variation in incident radiation that the sensor can detect (i.e., how small a temperature variation can the imager see). The sensor can detect temperature variations in things that are too small for it to measure.
Each sensor in the array will have a different response curve to infrared radiation. Each cell in the array will vary in signal response from other cells around it for the same incident radiation. This is corrected through the camera firmware (set during the factory calibration process) and is periodically normalized through the NUC (non-uniformity correction) process, which corrects for drift. Most current imagers NUC automatically.
Basically, your imager will periodically flip a uniform temperature screen up in front of the sensor and readjust for the signal level of each pixel in the array so that it renders the same temperature across the whole array. Think of it like the mirror in an SLR camera. Every time your imager freezes and makes the familiar clicking noise, which is the screen flipping up in front of the sensor, it is going through the NUC process.
The NUC process is what eliminates the “smearing” that you refer to.
This is also the reason that your uncooled imager should be given time to warm-up and reach equilibrium in the environment in which you are using it. You may also notice improved image clarity an accuracy immediately after the NUC event.
This is but one of many reasons that thermography is not simply point and shoot. If you are doing energy audits you should be focused on the thermal patterns (qualitative analysis)
Yes you can see significant differences between imagers with different spacial resolutions and thermal sensitivities.