thinking out loud in a mix of what I think I know and pure conjecture here...
The actual spacing of plain cloth woven fibers is, I'm sure, way too large to prevent a typical(?) virus from passing thru, whereas the fiber/weave spacing in 'good' masks is much smaller, and will stop most viruses. --I assume--
Therefore, using actual ratio's of virus size to the spacing of the "holes in the fence" to prevent it from getting thru, then I can believe studies that say only 5% of viruses can be blocked by the "fence"; 95% of them will fit thru the holes, while 5% are too big or will get trapped.
Speaking of trapped; a plain cloth "fence", has lots of stray fibers surrounding the woven 'holes' that a virus could get trapped in. And it seems only logical those fibers WILL trap many viruses, if they actually look like the pictures they show us, with all those little protein 'hooks' sticking out and all...? This might account for another study showing a much higher rate of blockage...
Now factor in that the Covid-19 virus is heavy enough that a lone virus will fall to the ground (saw a doctor say that ), and has no propulsion system other than air movement, either ambient, or that created by us, by inhaling and exhaling. Seems logical to me that a virus trapped precariously by random t-shirt fibers, has just as much chance of being blown OFF the mask via exhaling as it does of being sucked thru the mask via inhaling...? And, if it IS dislodged by low-velocity exhaling, because it's heavy it's likely to just fall to the ground rather than be shot across 6' of airspace as a sneeze would do...
Even if all I just said is complete BS, Occam's Razor says 'the most obvious is usually correct'-- and IMO, any mask, even a 5%er, is better than no mask...