Hi Barry,
% complete is a duration measure - not a work measure. Have you consumed 50% of the overall project duration? Try adding the Project Summary task (Gantt chart tools format ribbon).

If you look at the project summary task - does the finish variance show 6 hours? Do you have tasks that are incomplete in the past? It appears so - you should not have task with zero % complete in the past that will certainly throw off the finish variance.