HELLO AASIF BASHIR..
I completely agree with you and it is an important point you have made here..actually i myself get confused sometimes when this happens it is not only with changing frequency.. i have seen this happening with power conversion also.. if you change power from dbm to milliwatts you get one value and when u convert same result back to dbm , there is variation by some decibels but it is manageable in that case..
bt in case of frequency to wavelength conversion it is a big issue since even .1 implies to huge values..
Thanks & regards