If I understand you correctly, the subtotal is just the sum of all the detail lines, so when you remove the detail portions, the line that is reported is the subtotal, it just does not get described as that, it takes on the name of the level that you are hiding.
Look at the value of your subtotal, then hide the detail, and you will see that they are the same value.
Alternatively, it could be that your subtotals are turned off. When you right click on a level, go to Field Settings, and you can turn subtotals on or off, select a different summary (min, max etc..) and even sort by a field or select how many records to show (top 10, bottom 10 etc...)
While at coffee the thought occured to me that you may be "Hiding" at too high of a level. Taking Mark's example 1 step further, if you had additional information that detailed the months by week (ie: 1,2,3,4) and you included that data in your pivot table, then to Hide the Weekly detail, you would click on Month. The value in the body of the table is your monthly subtotal now rather than the weekly detail.
If you wanted to just show Fruits, click on Month and Hide it. The body of the table is now the subtotals by fruit.