Why are my routes different distances on the gps?

[cloudgazer]

Using base camp (southern Africa street maps 2009)
On the computer my routes displays as a particular distance, but it's different when I've sent it to my zumo 400. Same distances ar the same but many if not most are off by a few kms (anywhere between 3-35 or so)
Whats happening here?

 

[iamgigglz]

 

[Moondog]

 

[Jacko]