Increased Manhours when Importing from MSP to P6

When I import a plan from MSP to P6, the work shown in budgeted labour units increases by approximately 1500hrs. I've tried changing the importing settings as well as the calculation settings on the project but it doesn't seem to make any difference. Initially I thought the Remaining labour units were correct however this is also not the case, they are less than BLU, but only by 60hrs. 

I believe the issue is something to do with the units/time on import from MSP, but I'm unsure how to fix this.

Any help would be appreciated. 

Thanks