But why shouldn't a single slice contributed by each member be sufficient?
R. Jose b. Saul citing Rabbi replied: "On account of possible ill-feeling."
That is, one resident might bring a loaf, others might bring a slice – and resentment could arise.
Well, it seems we could take care of that easily:
Said R. Aha son of Raba to R. Ashi: "What then is the law, where all the residents contributed slices [of bread to their eruv]?"
He replied: "There may be a recurrence of the trouble."
We start with everyone bringing a slice – but what if people in other neighborhoods, for example, are bringing whole loafs? Now there could be resentment among neighbors. No – better to keep the contributions of everyone equal.
Ok, this is kind of a silly example. But it is an interesting principle: keeping peace by being consistent. And keeping things whole.