That's because your microphone is not a stereo microphone but a monophonic source.  So, when you record a monophonic source in a stereo track, it will record to only one channel.

To record a true (centered) monophonic track, in the audio recording dialog box, you will need to go into the Advanced Options and choose the option to record monophonically.  That "should" give you the results you're looking for.