Any possible explanation of why a router's LAN connection routinely pushes more traffic than its Serial connection?

[CSCO10203269]

We have a Cisco 3845 we're monitoring with MRTG.  The traffic summary page routinely shows the Ge0/0 interface pushing significantly more traffic than its S1/0 interface; WAN connection is a DS3.  Looking at the stats today:

Interface Ge0/0

In: Max 9.43 Mbps, Avg 742.63 kbps, Last 1.66 Mbps

Out: Max 8.75 Mbps, Avg 523.94 kbps, Last 1.65 Mbps

Interface S1/0

In: Max 2.65 Mbps, Avg 139.24 kbps, Last 181.32 kbps

Out: Max 3.68 Mbps, Avg 355.81 kbps, Last 194.98 kbps

I was about to chalk it up to a MRTG misconfiguration, however, we have another location with identical hardware and circuit size that shows a discrepancy of a few 100kbps between the interfaces.

[John Blakley]

Do you have multiple subnets on the ge0/0 interface, router-on-a-stick config, or anything like that?

[CSCO10203269]

There is a secondary IP address on the ge0/0 interface the provider uses for management, however, that's consistent across all our routers. Aside from that, the other difference is apparently its doing encryption on the serial interface:

interface Serial1/0

ip address x.x.x.x 255.255.255.252

dsu bandwidth 44210

scramble

no cdp enable

max-reserved-bandwidth 90