We've got an intermittent issue with the calculation of labour cost when invoicing.

I've written up an invoice for a job with 8.5 hours labour at $70/hr which should be $595 before tax. We noticed the next day when we added the hours from our mechanics timeslips that there was a discrepancy in the originally invoiced labour cost and the labour cost calculated with the timeslip times.

The originally invoiced labour quantity was correct at 8.5 hours, yet the calculated cost was $525. When the timeslip activities were added, the labour quantity was the same at 8.5 hours, but the cost was calculated correctly at $595 causing a discrepancy of $70

Usually when we're invoicing we will enter the recorded labour before the job is done to save time, and then update the labour quantity when our mechanics bring in the completed work order for a finished job. This has never been a problem, and calculates the cost correctly when quantity is updated every time except for 2 invoices in the last 3 months. 

Has anyone else run into this, know why it happens, or know how to correct it other than manually checking all of Sage's calculations?

Thank you