Mash acid additions too high?

I love it that BS3 now has the water treatment and mash pH tools built in and I do not have to use an additional software tool to calculate additions.  Having said that, my mash acid additions have been coming out too high and this has resulted in the Mash pH being too low.

In my current recipe (an English Best Bitter), I decided to compare the recommended acid & mineral additions in BS to the tool I have been using - Bru'n Water.  I entered the same grain bill and Lovibond numbers and the same water profile.  I set the target mash pH to 5.3 in both programs and get the following:
BeerSmith:
Mash: 
  5.7mL Lactic acid
  1.46g Baking soda
  1.22g Gypsum
Sparge:
  1.9mL Lactic acid
  1.93g Baking soda
  1.62g Gypsum
BeerSmith indicates these additions should bring the un-adjusted water profile from 5.7 to 5.3pH

Bru'n Water:
Mash: 
  3.6mL Lactic acid
  1.5g Baking soda
  1.2g Gypsum
Sparge:
  1.8mL Lactic acid
  0g Baking soda (Not recommended)
  1.6g Gypsum
Bru'n Water indicates these additions should result in a mash pH of 5.3

Has anyone run into this before?  Any idea why BeerSmith is recommending 60% more Lactic acid?  I have had some recipes that BS calls for 8mL of Lactic which seems a lot and then ended up with a mash pH of 4.9.

Paul

Please see the sticky post on the pH model BeerSmith uses in the all-grain/advanced section.  Short answer is that the model over estimates the buffering capacity of the system.  The developer of the model has been collecting data to rectify it.

Very good.  I read the sticky and the HBT thread and these numbers agree with what I am seeing.  Changing the acid concentration to 145% to fudge the over-estimation seems to get the acid into the range I'd expect.

Thanks for directing me to the sticky.  I should have looked there first!

Paul