Covering the frequency range from 250 kHz to 20 GHz is not easy task with simple LO.
In my view, Agilent using two synthesizers one upto 250MHz and other after 250MHz.
So for this range LO needs to be sized by the sclae factor 1/16, Similarly for the double of frequency range i.e. 500MHz to 1G it will be multiplied by two i.e. 1/8.
If you still need confirmation best way is to talk with Agilent technical people.