So everybody knows that pedals are opposite threaded, so that the rule for either pedal to put them on is, just hold the spindle and freewheel backwards; or to take them off, grab the spindle and pedal forwards.

And yet I've read that riding (pedaling forwards) doesn't cause pedals to loosen, but to tighten (such that they gradually even dig in to the metal of the crank, which I've seen the evidence of)

Similarly, an english BB fixed cup is reverse threaded, so it's rightie-loosie. Which would seem to indicate that pedaling forward would tend to loosen it -- but I've also read (and experienced!!!) that a fixed cup will get ridiculously tight with use.

Can somebody explain this to me?