Well in case of addition and subrtaction, it is differnt. What you are saying is if they contain 4 digits then it is supposed to be the same in the answer, but this is the case of multiplication and division. While in case of subration here in this case it dfferent, the answer is 2.5 (having 1 digit after the decimal) becasue the 2 numbers that you are subtracting from each other are also the same..
So, hope it helps a bit
i can give a further example that might help you more,
30.26 - 1 = 29.26=~ 29 because the least precise is the 1 so you round as it..
So hope now it's better..
Good Luck:)