IBU differences on web based calculators

I started brewing several months ago and just made batch #6 (Karmeleit klone). My first two batches were extracts following a pretty well set forth recipe. I then switched to all grain and taking recipes off the web and butchering them for my own effect. I found beer calculus and I like it a lot (see my recipes under patrickmathews). I was looking around for some other mash calculators and decided to put my hop bill in rooftrop brew IBU calculator - here are the results for both.

Hop bill:

2 oz Tettnanger 60 min pellet 4.5 AA
.5 oz Saas 5 min leaf 3.5 AA

beer calculus - 29.4
Rooftop Brew - 38.3

Any idea why such a discrepency? Any idea which is closer to reality? 29 was at the upper end for what I was trying for with this beer. If I end up at 38 - well it won't taste like Karmeleit.

They use two different equations for calculating IBUs, so the results are going to be different. If you've consistently used a certain equation before, use that one. Otherwise, you're probably best off using the beer calculus numbers because the Tinseth equation on beer calculus is the equation most people use.