It is very probably a rounding difference dues to adjustments/attendances being applied.
First check for errors:
Go to Tools, Check Job, OK.
If errors are found and cleared then re try the update from comparisons
If the Project is clear of errors try running Update Comparisons without the Adjustment and Attendances activated and the Gross total should match under the Analysis, Estimate, Job Resources list.
If the Gross total in Comparisons (without adjustments applied) matches, the issue is caused by the attendances and adjustments being applied.