MATLAB Data Acquisition Toolbox + PCI 6010: startBackground vs. singleInputScan timestamps

For the same type of analog signal being input into an analog input port of NI PCI-6010, timestamps returned by startBackground are way off; whereas singleInputScan seems to return the correct timestamps of the samples. Does anyone know why the timestamps from the 'DataAvailable' callback routine called after startBackground are off by like 1 second. Shouldn't the timestamps reflect the instant at which data was acquired at the analog input port and not when read by the MATLAB routine? How can one get the actual timestamps while using startBackground?