Multiple yeast strains- Estimated ABV changes unexpectedly

I intend to brew a very high gravity beer (around 15%avb). I plan to use multiple strains to accomplish my goal. I will start primary with two Belgian strains that will impart my desired flavor characteristics and finish with WLP099 for attenuation.
Now, when I enter these into beersmith, it estimates my abv to be 15.3%. When I save and come back to the recipe it shows 14.2%. I thought that changing the order of the yeasts in the recipe would change something, but it appears they organize by alphabet. Shouldn't it calculate based on highest degree of attenuation?  Why would it change when I save?

For a single yeast, I 'thought' that BS2 uses the mid-point of the attenuation range for that yeast.  For ex, AA% range is 70 to 78, so it uses 74%. 

Not sure what it's programmed to do with multiple yeasts, but you might try duplicating the number it gives by doing those type averages. 

How long did you plan to let the Belgians go before adding the WLP?  Once there's a lot of alcohol and no oxygen left, if's tough to get another yeast started.

Thanks for the tip. I oxygenate when pitching the second strain. Second strain would be added once approximately 50% attenuation had been reached with the character strain. Honestly, It's probably unreasonable to ask BeerSmith to calculate all of this. I can just learn by trying and documenting.
Actually tried this technique. Worked very well.