One explanation I can think of is the difference in reported %AA on the hops. BeerSmith uses a default value that is typically in-range for the particular hops, and this default value can vary from what your package may say. It is a good practice after procuring your hops to touch-up your recipe by entering the exact %AA as shown on the package. This ensures that, at least, you will be entering into BeerSmith the actual %AA you will be putting into the pot.
Another explanation could be the timing of the hops addition(s). The earlier you add your hops, the more utilization you will get and BeerSmith will show this in their calculations. Move your hops around with respect to time and you will see your IBU value move along with it. When copying a recipe, be sure your timing is adjusted to match their timing for hop additions to give the calculators the best possible chance of arriving at the correct IBU value.
Another possible explanation is the particular recipe's method of reporting its IBUs - there are a few methods of doing this, among them are Tinseth and Rager - each with its own reasoning and slightly different means of making the calculations. Some recipes are published using Rager for reporting the IBUs, some use Tinseth. Each will give you a different value when formulating your recipe. While each method has its merit, most folks tend to just pick one and stick with it. No one method is "technically better" than another, it's just a matter of what works for you. BeerSmith defaults to Tinseth. Try going into BeerSmith and select the Options button located just above the recipe field and hit Bitterness in the left column, now change the Bitterness Formula from Tinseth to Rager and see if your recipe looks more like what you expected.
Any one of these possible explanations could be the key to your recipe's IBU inconsistency - play around with these adjustments to see what works for you.
<edit> I ran the numbers from Jamil's recipe, which was done using another brewing program, and sure enough; it was done in Rager. When I put it into BeerSmith, I got 14.6 IBU (after adjusting the %AA to 13.5 as in Jamil's recipe). I then changed the Bitterness Formula to Rager and got 18.1. Note - you will need to change screens to get this change to take affect in your recipe. You can just click the Starter button above your recipe field and then back to Design and you should see the change in IBUs.