Each time Lansweeper scans a computer, it makes an entry in tblCPlogoninfo if any user is currently logged on to the computer. This table is being cleaned up according to your settings under 
Configuration\Server options, section 
History Cleanup Options (Delete logon information after X days).
If a user was logged on at a time when the computer wasn't scanned, Lansweeper won't get to know it. If you would like to collect precise logon information, you need to configure 
LsPush scans in logon scripts.