Let me start by saying that the term "true resolution" has no set meaning. It is a term that Snapsort uses to try and simplify the meaningful detail a camera can capture.

Resolution, at its most basic, is the level of granularity of detail that a camera can capture. You could have a 200 mega pixel camera, but if the image was out of focus and you only had a giant brown blob, the meaningful resolution would be pretty much nothing because you can't make out any level of detail.

A large number of factors impact how much detail you can capture, the quality, speed and clarity of the lens, the positions within the frame that you are looking at (the center is generally higher detail than the outside), the size of the sensor (and consequently the diffraction limit), the level of noise on the sensor, even atmospheric conditions can impact the total level of actual meaningful detail that can be captured by the camera.

"True Resolution" is simply Snapsort's attempt to generalize that down in to an easily consumable number, but as it is a gross over-simplification of a complex topic, it is also next to useless. For example, one cheap lens may have a super sharp center but fall apart near the corners. It would end up with a low total resolution because of the average, however another lens that is generally uniform but fairly low quality might end up being marked as higher "total resolution".

The problem is, if you are taking a portrait with the subject in the center for example, that you may not care about edge sharpness since all the detail you want will be in the center. Thus, the "lower resolution" lens would actually be the better choice.

Snapsort is a good source of basic stat comparison's between cameras, but a large amount of their information is overly simplified and thus useless. Don't put a lot of stock in their comparisons as they are not particularly trustworthy or reliable.