Exchange 2000 backup

I am running Exchange Server 2000 and regularly back up the Information Store to a second disk on the exchange server using the Microsoft Backup utility included in Windows 2000 server. I have 35 users on the server and the backup file is approx. 900 MB. I run the backup in off hours and everything seems to be working fine. The backup usually takes 45 minutes to an hour, but occasionally will run in 6 to 8 minutes. The output file size looks good and the log shows no errors. Does anyone know what might account for the variance?