With percentage you express portion, but 5 out of 4 is as real as 125% is real in some situations. For example you can get an extra hour on your laptop that would usually last no more than 4 hours by lowering display brightness. You got 5 out of 4 hours.
5 out of 4 sounds unnatural, but so are negative or complex numbers in most measurements that you can make in nature - you can't have -3 stones, your can't have a negative momentum, but that doesn't prevent us from using negative numbers when we feel like it.
5 out 4 is only another representation of a fraction, which is mathematically not problematical, it's just that in physical world many things can't go over 100%.