Difficult to Guess What is going on without some sample.
I'm sorry I'm unable to provide a sample. If you have a suggestion for replicating this setup so I can share it, I would be happy to hear them.
If you create the extract without aggregation can you do the right calculation ?
I've never seen this issue, but I think it might be a case of what you expect to happen is not what Tableau does.
I just finished creating an extract without the aggregation option. The views are exactly as expected, so there must be something about this checkbox I don't understand.
Thanks for the suggestion!
Awesome at least it's working in some regard.
Now the average and aggregation option means that it will summarise your data and average the values on aggregation while it's creating the extract.
Without seeing both the SQL and the extract, I'd guess that your SQL is not aggregating the data and then averaging whereas your extract is.
Depending on performance, you might just leave it with no aggregation or play with it more to fully understand what's really going on.