Yeah, I finally found some place with a datasheet that did some explaining. It's to deal with the transient readings that will occur when the environment is mid transition between two steady states. When the environment has reached steady state, the reading will zero out but while it is in transition there wil be a reading.
The ones with two or more side-by-side are to cancel out the changes in temperature that fill the sensor's entire FOV (things like ambient changes). They subtract the AC signals of the sensor elements from each other so only a differential change between the two elements will produce a non-zero output. So something like a moving warm body will pass in front of each element at different types and produce a reading, but a large wall that is heating up will be seen by both sensors at the same time and produce zero output. Elements arranged in a linear array will make the sensor only pick up thermal changes that move parallel to the linear array, while elements arranged in a grid will cause the element to pick up thermal moving in any direction.
Others have a smaller die in the middle and on top of a larger die to cancel out the readings due to thermal changes of the packaging itself. These ones will still pick things up like the change in the temperature of a large wall filling the entire FOV. These kind seem to be more for temperature or energy measurement than motion sensing.