Usage difference between LAN and WAN

Hi,

I’m using RB33AH.

I have configured SRC NAT and Mark Routing.

WAN: 41.x.x.30/27
LAN: 41.x.x.129/26
172.18.0.1/24
172.30.0.2/24
IP address.JPG
I have configured SRCNAT:

0 chain=srcnat action=masquerade src-address=172.18.0.0/24

I’m using Mark Routing and configured mangle:

0 chain=prerouting action=mark-routing new-routing-mark=All NAT Clients
passthrough=no src-address=172.18.0.0/24

1 chain=prerouting action=mark-routing new-routing-mark=Is-Nigeria Pub
passthrough=no src-address=41.x.x.128/26
IP Routes.JPG
But most of the time I see there is at least 1 Mbps more usage into my WAN interface.I have taken 8Mbps BW from ISP.Sometimes my LAN usage is 6.3 Mbps but I see WAN interface showing 8.0 Mbps.As a result, I encounter PKT loss.
Interface Usage.JPG
I’m not using Web Proxy. What could be the issue? Any idea ?