Smuttynose Old Brown Dog Ale, bitterness confusion

So, right from Smuttynose's website:

Hops
Bittering: Cascade and Galena
Flavoring: Willamette

Starting Extract 15° Plato
Terminal Extract 3.26° Plato

ABV 6.7%

IBU 18

I'm confused, not only do they list Galena as a hop used, but they list it as "bittering" - 60 min? I mean, isn't Galena way too high of AA to yield 18 IBU if used as a bittering hop? Especially in conjunction with Cascade as well...

Plugged into beersmith, even ONE TENTH OF ONE OZ of Galena would make the beer 24+IBU.

I'm confused. Still somewhat new to all this too.

My guess is that the IBUs they are listing are measured, not calculated. All the formulas for calculating IBUs are imperfect since yeast, time, etc can pull IBUs out of suspension and process can effect how many actually get absorbed into the wort in the first place.

I'd go a few IBUs over, shoot for 20-22, and taste it side by side with an Old Brown Dog when it finishes. That way you can compare the bitterness at least. I'd also switch to grams for hops so that the amount doesn't look so small , and its easier to measure more accurately on most scales.

OBD is a great beer. One of the first that got me into craft beer

Do you think it's possible that since their volumes are so tremendously high that they are putting in what would equate to like, a single pellet for me?

I used the Rager Formula and for 1.061 wort i came up with ~8g cascade and ~6g galena for 21 IBUs. 6g and 5g will get you to ~18 IBUs.
I really think this measured vs calculated issue.

A few grams for you is probably a couple pounds on their system, so they have the ability to use smaller quantities of high alpha acid hops to bitter their beer. They also can send their beer out to be measured.

I'd use OBD from the store vs a clone you brewed so that you can get an idea of how measured IBUs taste vs the calculated IBUs you get from the formula you use.

Yea. The volume thing makes sense, but it just struck me as odd that if they use such a small relative amount that it would be a large enough amount relative to their volume to make enough of a flavor difference.