Objects which fall under an equivalence relation are "equal", but they may
not be identical. Perhaps Marx is relying on this when he says that the
relative form of value is opposed to the value it is compared with as
exchange value to use-value. These two values will be equal, but as Marx
suggests, they may not be identical: in "the value of X equals the value of
Y" we can be talking about two entities, the value of X and the value of Y,
ie, they differ in what they are values of, even if they are equal.
The value relation is itself an equivalence relation (requires for its
production the same expenditure of socially productive labour).