Yeah, I ran across something similar once... I have literally seen the attendant number change after a delete and re-add as was evident by seeing the attendant hold remote retrieve code change (you must enter the console hardware number as part of the dial string), I will apply that knowledge here.
My guess is there maybe one or more things going on here. It is not uncommon in Mitel systems for a device to get an ID that never changes, and doesn't get reused, which maybe why there is no ATT6. Perhaps consoles were 1-8, but before the system was placed in service they noticed they messed up something on one of the consoles so they had to delete and recreate it (or even build a new one, then delete the old one after using it as a template), or 6 may still be a database "phantom" tying up a software ID under the hood of the software in the database. No an uncommon scenario and would explain why there is no records for ATT6.
Extending that concept a little and adding a tidbit of 8.0 knowledge, it is quite possible that ATT10-ATT16 were the numbers up until the upgrade... when upgrading to 8.0 there was a frequent bug that caused Users and Device to be blank after the upgrade even though all devices worked fine. The only solution was to backup and restore the database, causing the corrupt data to not be restored and everything to be right with the world again in the ESM console. This process can also have some unusual side effects, like the hardware ID's of IP devices being renumbered to the restore order, which usually has no effect on most things, but may in this case.
Not sure if I am right or not, but it is plausible... But it also means it may not be feasible to get reports from before and after the upgrade and combine them. Or I could be way off base too.