CW sending speed. A zero takes the most time to send, so they pick a call with a 5 or 4. At 25 WPM it takes nearly 1 second to send a zero, where it takes less than half that to send the number 5. Over the long haul of a contest you can work more stations by reducing the needed time for calling and exchange. For me in 4 land, where competition for any extra class call is fierce, I didn't want to spend what time on Earth I have left waiting for something that I even halfway liked phonetically to become available and then lose the lottery and start over again and again. When I was in the running even crappy calls had upwards of 30 applicants. I was the one and only applicant for this call.