[mmorse]

If no full backup for an account is found, the backup process performs a full backup on this account, even if only an incremental backup was specified.

For new users there will be no existing full backup; so you're fine if the accounts listed in the 'random' fulls are the recently added accounts.

By default:
Full backup is scheduled for 1:00 a.m., every Saturday.
Incremental backups are scheduled for 1:00 a.m., Sunday through Friday.
(check your zmschedulebackup -q and crontabs)

Look at the below 2 examples:
-You've simply added a user on the day of the two bold thrusdays below going from 52 (drop 1 & add 1 on thurs?) so back up 52 then to 53 users:
-Italics is a normal full on a saturday.

Type: incremental
Started: Fri, 2008/05/09 01:00:23.409 EST
Accounts: 53

Type: full
Started: Thu, 2008/05/08 01:01:35.338 EST
Accounts: 1

Type: incremental
Started: Thu, 2008/05/08 01:00:19.234 EST
Accounts: 52

----snip---

Type: incremental
Started: Sun, 2008/04/13 01:00:21.593 EST
Accounts 52

Type: full
Started: Sat, 2008/04/12 01:00:13.311 EST
Accounts: 52

Type: incremental
Started: Fri, 2008/04/11 01:00:33.542 EST
Accounts: 52

Type: full
Started: Thu, 2008/04/10 01:01:35.256 EST
Accounts: 1

Type: incremental
Started: Thu, 2008/04/10 01:00:27.421 EST
Accounts: 51

Type: incremental
Status: completed
Started: Wed, 2008/04/09 01:00:20.844 EST
Number: 52

Notice that wed at the end - Did you remove one account then add another on thurs?
Wed: 52
Thurs: drop 1 & add 1? (52)
Fri: 52

Either your getting new workers/doing account maintenance on Thursdays or there's something scheduled on one account somewhere? (Do the 1 account's omitted match?)