autosum in excel 2007

Hi
Billy 27 here asking for help about autosum.
I have recently noticed while using one of my templates for keeping records of one of my pastimes some thing that happens in some cells but not others.
If I place an amount such as 12.00 in one cell then 12.00 in the next cell and then highlight the next cell and click autosum, the result 24.00 appears, which is what I would expect. However, in some cells further down the work sheet, if I do the same thing, ie enter 12.00 in one cell then 12.00 in the next cell and then highlight the next cell, the result, 24.00, appears without me clicking on autosum. How does this work and can I set my worksheet up so this happens in all the cells?
Thanks to any one who can help me.
Billy27