Steve,
Straight from the Mib. Looks like only traffic via the software path increments the counter. For hardware switched traffic, this counter will not be incremented.
I work purely with SNMP, so not very sure on the architecture of those 2 platforms and how it applies here. You might want to post this to a switching alias, or open tac case.
.1.3.6.1.2.1.4.6
ipForwDatagrams OBJECT-TYPE
-- FROM RFC1213-MIB
SYNTAX Counter
MAX-ACCESS read-only
STATUS Mandatory
DESCRIPTION "The number of input datagrams for which this entity was not their final IP destination, as a result of which an attempt was made to find a route to forward them to that final destination.
In entities which do not act as IP Gateways, this counter will include only those packets which were Source-Routed via this entity, and the Source-Route option processing was successful."
::= { iso(1) org(3) dod(6) internet(1) mgmt(2) mib-2(1) ip(4) 6 }