From the Net-SNMP site regarding snmpget (Sourceforge):
"A common mistake when using the snmpget command is to leave off the index into the data you're looking for. In the above commands, the variable requested by the OID is a scalar and the index to scalars is always a simple '0' (zero), hence the trailing '.0' in all the oids above. If you had left it off, you would have gotten an error."
If you're using snmpwalk (or some equivalent) you're asking for anything at or below the OID you specify, snmpget ordinarily has to have the complete OID of the actual value.
Other implementations may behave differently, of course. But this is (IIRC) the official behavior.