Sooo... to dumb it down even more, some of the RNA changes have made this strain more able to infect enteric cells (digestive tract lining), so there will be more of that strain than others there - and those are the cell remnants and shed virus particles that end up in wastewater. Said more simply, people who get this strain end up pooping more virus than people with a different strain - so wastewater monitoring sees more of this one.
Nice explanation of synonymous vs non and how that can affect molecular shape, why that might matter BTW. Thanks!