The manual says: If a packet does not match any rule in any context on the firewall, the Global Reject or Global Drop rule drops the packet (Global Drop) or drops the packet and sends the appropriate reject message (Global Reject) even when the system is in a default allow configuration.
If the counter for Default rule is not incrementing but they are incrementing for Virtual Server or the Self IP, that means there are more specific matches and not hitting the default rule on Global context. Packets dropped on Virtual Server or the Self IP context will not have an affect on the Global counter.