Hi Kirk,
If you look carefully at the GSV message and read the fields, you will notice the SNRs of the satellites are the following:
<blank>,<blank>,<blank>,<blank>
<blank>,<blank>,<blank>,<blank>,
42,<blank>,30,<blank>
<blank>
Unfortunately the reciever sometimes reports SVs that it does not have a signal on. Interference, in addition to signal level to the chip could also be a problem.
Regards,
Aaron