I have seen a lot of this when converting to DXF and it can't always be explained. Verify all DXF output settings are correct for line type, scale, etc. remove any unnecessary lines from original DWG, ensure you are not exporting unwanted or "hidden" layers. it is much easier to edit the master file rather than working backwards from a DXF. Best advice I can give is to open and verify your DXFs before sending them out to the shop.