my log analyzer and bytes_sent are not matching

I'm trying to use mod_log_sql to calculate daily bandwidth usage stats
for clients.  I'm basically using this:

select sum(bytes_sent) from `access_domain_com`;

I'm also using awstats for log analyzing.  This is how it works.

A script uses  make_combined_log.pl:

 /usr/webservers/src/mod_log_sql/make_combined_log.pl 1 domain.com > 
/var/log/httpd/domain.com/access_log

redirects output to a file.  This file is then used for awstats to create 
pretty log stats output.

Using another script I grab the sum for the same domain and put this sum 
into a database.

The sum from mod_log_sql is always much higher then what awstats comes up 
with using the same information.

What could cause this?  I really need to figure this out because customers 
are either getting charged too much or too little.

Thanks
-jeremy