Stuff in my exhaust

Hi People, hope you are all well.
Over the last few weekends I have noticed ash like material in the left hand exhaust pipe after the initial fire up. I can only describe it as ‘bonfire ash’ that crumbles in your fingers. Once it has been running and cleaned out (ie warm) then it does not return so it must be something to do with condensation after being left for a couple of days, but why only the left pipe ? Any ideas ?

Do you have a standard exhaust fitted? If so I can’t explain why this is, however if you have an aftermarket version the pipe from the silencer comes out of one end, loops around, splits in to two and exits in the usual place, the way the pipe bends and the pressure and speed of the gases mean that they are forced out of the left pipe.