Stir plate effect on yeast starter numbers

The new Beersmith2 has a nice feature that can calculate the starter size required using a stir plate.  I have been using Mr Malty’s calculator for some time now and there seems to be a very big difference in the stir plate effect between Beersmith and Mr Malty on this.  For instances: both programs suggest we need around 13.5 billion cells to ferment a 125L batch of 1.060 of ale.  But Mr Malty suggests 5 packs of liquid yeast and a 7.7L starter when using a stir plate, and Beersmith suggests 2 packs and a 6.55L litre stater when using a stir plate.  Can anyone throw any light on this?