There is a little of that but we are talking millions of credits difference. I think it is due to the way retired projects are being handled.
For example:
http://boinc.netsoft-online.com/get_...173c6d1cf16b44
Total Credit: 107,133,764
http://stats.free-dc.org/stats.php?p...173c6d1cf16b44
Total Credit: 109,665,865
Difference: 2,532,101
Now on the other hand my personal stats are pretty close.
http://boinc.netsoft-online.com/get_...314dcc2a857cb6
Total Credit: 1,303,635
http://stats.free-dc.org/stats.php?p...314dcc2a857cb6
Total Credit: 1,303,643