When I look at my
Sale>Orders>View All screen 
in General View, some of my orders appear to have an incorrect status showing against them. Why is this?

If sales are created using advanced sales module, statuses will be calculated differently compared to when a sale is completed through the standard sale module.

Due to multiple fulfillments/invoices that can be performed per sale order, status can go from Ordered/Backordered to Completed only in General View. You need to use a different view for example Accounting View or Fulfillment View to see the exact progress of sale created through the advanced sale module.