arecord for single channel still stereo

I have a BB verions B5 board with Linux 2.6.22.18-omap3 from:

http://code.google.com/p/beagleboard/wiki/BeagleSourceCode

and ALSA version 1.0.15

When I try to record a single channel, such as:

arecord -t raw -f S16_LE -r 8000 -d 4 -c 1 -v test.raw

the recording is still in stereo, but for only half the duration (in
the above command I get two seconds of stereo recording).

How can I record a truly mono file?

Thanks.