EDIT: From the linked Wikipedia entry: "Because of the much greater distance to ground, the positively charged region can develop considerably larger levels of charge and voltages than the negative charge regions in the lower part of the cloud."
Also, having Cloud to Butt extension installed made reading that Wikipedia quote quite interesting...
EDIT: From the linked Wikipedia entry: "Because of the much greater distance to ground, the positively charged region can develop considerably larger levels of charge and voltages than the negative charge regions in the lower part of the cloud."