They are the same number.
8.4 is 8 units and 4 tenths.
8.40 is 8 units, 4 tenths and zero hundreths.
So, you've actually added nothing! Indeed, you can add as many zeroes as you like at the end of the decimal expansion:
[tex] 8.4 = 8.40 = 8.400 = 8.40000 = \ldots [/tex]
