Some temperature difference between the edges and the center is normal, but that sounds like a larger variation than you should have. Have you tried letting it sit hot for a while to let the temperature even-out before measuring? It may also be that the non-contact thermometer is not reading accurately, as the D:S ratio (these devices measure an area, not a single spot) and peculiarities of emissivity can result in inaccurate readings. For example, if you had the dot pointed exactly at the corner of the platform, even if the platform's temperature was completely uniform, the I would expect the non-contact device to show only 25% of the difference between the platform's temperature and ambient, because 75% of the area it was measuring would be measuring would be outside the platform. In order to get an even remotely accurate measurement with one of these devices, you should have it almost touching the surface, and the distance from the sensor to the surface should be more than the distance between the center of your measuring area and the edge of the platform.