is your expression? Not sure if I would sum averages, but you will know.
You could try using advanved aggregation to get an average of your table values instead of an expression total.
=avg( aggr( sum(Average) , Product,Branch,SalesMonth, Stage))
Please check the Help for some information about using aggr() function in the context of sum-of-rows (which in your case would be better called avg-of-rows).
If the level of aggregation is not correct, you could probably succeed with adding a total qualifier or refining the aggr() dimension list / using two embedded aggr() functions. If you are running into problems, could you post some mockup data together with your required table outcome?
Hope this helps,