Measurements such as Insertion Loss, NEXT and Return Loss are referred to as "swept frequency" measurements, meaning that certain points between the minimum and maximum frequency of the selected test standard are measured. It goes with out saying that if you only measure 100 points between 1 MHz and 1.000 MHz - it will make for a very quick test, but fail to provide the desired resolution.
So the field testing standards ANSI/TIA-1152 and IEC 61935-1 define the minimum frequency steps (resolution) for the test equipment vendors.
IEC 61935-1 Ed.4
The DSX provides better resolution than the minimum required for IEC 61935-1 Ed.4. It is worth investigating what resolution your vendor is using.