Why the two numbers of Total bytes of Ingress Router A and Egress Router B are not equal???

Hi forum,

My Company has two site BachHO (HO) and CTK3 (Branch). We connected two sites using an WAN. I monitored ingress and egress traffic of WAN interface of Router WAN at both sites. I suppose that in a specific of time the Total bytes of Egress of Router BachHO must be same as the number of Ingress of Router CTK3. But in reality, they are not same. I cannot find out the reason why??? Plz explain to me.

Here are some pictures

The two topmost pics are Router BachHO

The two bottommost pics are Router CTK3