John,
There a few things that could be causing your problems.
1. As you guessed, there is a chance you might see some "ghost" channels because there is only on ADC on the DAQ board. Normally, though, this discrepancy will NOT be 2 volts or anything nearly that large.
Check out this KnowledgeBase:
http://digital.ni.com/public.nsf/websearch/5B86A63ECC0D48828625629800571743?OpenDocument
2. The other problem could be with how you are wiring things up. Take a look at this link:
http://digital.ni.com/public.nsf/websearch/D509679FFAE2764386256297005D0C9D
http://digital.ni.com/public.nsf/websearch/1FFCBE3451ABEE09862562970059881D
Also, if these do not solve your problem, reply with exactly how you are wiring up your channels.
Regards,
Anuj D.