The reason is that In your model there are sales not associated to any year, the - you see means null values, in your chart in the dimension tab check the "suppress when value is null" box, you shoult even take a look to your model (How's that there are sales without year?)
In your data model, there might be products that have never been sold or customers that might have not placed any orders.
All the above cases give us a Null value in the Facts table as a foreign key (since there is not existing Order to associate with them). This null is not associated with any year, so it appears as a separate value in your chart. That's why you see this extra value.
You can suppress it by following the above instructions.