Level 0 export performance variation

We run the level 0 export via a batch script that calls a *.mshs script that first sets the app to an offline mode then we run the export as following:

EXPORT DATABASE 'HXXXX'.'BalSh' LEVEL0 DATA IN COLUMNS TO DATA_FILE 'c:\temp\Backup\Nightly\Lev0_HBalSh.txt';

The above statement is run for 10 cubes with the output resulting in up to 4 export files that are 2 GB in size.

We noticed that the export time for each cube varies from night to night.  One night a cube will export the same 4 files in 26 minutes and other days take 82 minutes.

What could cause this difference in export time for the same amount of data?