Hi Robert,
what you see is a common behavior on almost all DSBU platforms (3750, me3750, me3400 etc) while you get those types of stats on GSBU (Cat4k) and ISBU/ERBU (cat6k/C7600) devices.
SVI's input and output packet counters will not increment for layer 3 packet forwarding between
SVI interfaces in the hardware path. This behavior is
a platform limitation. Bug CSCsi44166 explains that for the 3750 but the same reasoning applies
for me3400 (same port asics). It is in fact
due to the architecture of the switch, collecting this info from
the ASICs would lead to a big performance impact.
For the same reason you would not want "to force "routed"/layer-3 packets to traverse
the CPU in order to get the SVI stats needed" as it will come with
a big cost that is software switching which is neither wanted nor supported.
Hope this helps
RIccardo