Everybody says that
"Serializable" means that for any given set of concurrent transactions, there is a serial ordering of those transactions which produces the same result.
They also give an example
suppose you have an attribute whose initial value is 10; and you have two transactions - one which doubles the value, the other which adds seven. Two different clients initiate the two transactions. Under isolated semantics, the only acceptable values for this attribute are 27 (the result of doubling first then adding 7) and 34 (the result of adding 7 first and then doubling). Any other value (such as 20 or 17) would be an error.
Which "same result" do we have in the example? Which reordering can we have to produce the same result?