[Source] 5 MPH over could be a discrepancy in measurement equipment, and officers do not want to go to court to explain when and how their radar gun was calibrated, etc. If they see you doing 20 over and write you a ticket for 10 or 15, it's less hassle for them. This cuts both ways: if your speedometer is slightly miscalibrated (entirely possible), you might be doing 10 over when you think you're doing 5 over.
Would someone please explain and elucidate this? If you're speeding by 20 (any unit of speed), why's it 'less hassle' to 'write you a ticket for 10 or 15'? Per contra, would this weaken the officer's argument? It's easier to argue based on a discrepancy of 10 or 15 rather than 20?