OK... here is my opinion. I am not a filter designer, but here it goes.
All the real phresh filters use RC-48 activated carbon... (I assume the same as medi's GP filters) This carbon should be relatively tightly packed so that the air is forced to touch some carbon at some point during its travel into the filter. The reason Phresh claims they cannot be reloaded is because the proper packing procedure and machines required to ensure that ALL the air passing through the filter actually touches the carbon. Anyone with half a brain can open the filter, dump the carbon and reload it with some carbon... This is not recomended and I'll explain in different words in a sec.
As for CAN filters, I believe they were reloadedable at some point. Since they have a larger "carbon cargo" area it's a bit safer to have the DIY guys reload the filter. the air is likely to touch carbon if it has to travel through 2" of carbon vs 1" for example. I do not know if those are the proper numbers. This also explains the extra weight of the CAN filters. They have more carbon of the cheaper chunkier carbon.
Why is RC-48 carbon (finely crushed carbon) better then standard fish carbon? Let's think of a tube 4" in diameter. Let's fill that tube with rocks... is it full? No, let's put some sand to fit between those rocks... is it full? For this analogy it is. The point is, the pourus rock in this tube by itself would allow air being pulled through it a probability that it sneaked past the rocks without touching them. If the finer rock (like sand or RC-48 carbon) was used then the air would be to guaranteed to touch carbon.
I'm happy to buy new filters every 4 runs. They are still good by most standards when I change them out.