Let's simplify this for you since you want to seem to rather find a way to clutch onto some percentage point that children suffer a higher ration of harm in foster care because there are less foster homes. So let's make this as perfectly clear to you as possible:

Who Are the Perpetrators?
In 2017, parents—acting alone or with another parent or individual—were responsible for 80.1 percent of child abuse or
neglect fatalities. More than one-quarter (30.5 percent) of fatalities were perpetrated by the mother acting alone, 15.5
percent were perpetrated by the father acting alone, and 20.2 percent were perpetrated by the mother and father acting
together. Nonparents (including kin and child care providers, among others) were responsible for 15.2 percent of child
fatalities, and child fatalities with unknown perpetrator relationship data accounted for 4.7 percent of the total.

No matter how much you'd like to twist it ratio wise the cold hard fact is this:

100% of abuse, neglect or fatalities originate in a child's natural home setting. I didn't need a study to tell me that, that's just plain logic.

When we go to the study, as document but somehow you have a reading comprehension issue when you hit the above paragraphs....

when it comes to fatalities 80%, let me repeat that, EIGHTY PERCENT, die at the hands of parents or a parent, now in case you are still having trouble seeing that number let me separate it from the rest of the words...

...all other fatalities combined is 20%. That is:

of the population other than a parent(s) who kills kids.

You can flip it, twist it, turn it, flop it, fry it in a frying pan and hope it sticks to the ceiling but the truth is when it comes to murders children are a greater risk from parents then the general population as a whole.