The basic explanation is vertical metres are accumulated whilst riding up a hill. The distance travelled is irrelevent, only the height of the hill is relevent. The steeper the hill, the more metres accumulated per distance travelled. Think of height from sea level to the top of a particular hill/mountain.
The hill I chose was just under 500m in distance, but climbed 60.6m in elevation (for cyclists out there ~13.3% average). In my time limit of 12hrs I did 153 laps of the circut (up and down the hill continuously) = 9271.8m. To put into perspective, Mt Everest peak from sea level is 8848m, so effectively I climbed higher than the height of Everest in a single ride, in 12hrs.
pinkboy