Concentration Skill Check - broken?

[Mindspat]

Please explain the following combat log associated with a Concentration skill check:

(Combat): Fiend-Blood Troglodyte Theurge's ball lightning hit you for 37 points of electric damage.
(Combat): You attempt to save versus ball lightning. You roll a 2 (+23): save!
(Combat): You make a Concentration skill check. You roll a 10 (+40): failure!

There was nothing other then the Fiend-Blood Trog Theurge to interfere with casting. I was stationary, shield blocking, and attemping to cast Cloudkill.

Currently, I notice all kinds of inconsistent results when required to make a Concetration check. For the time being I am concluding that Concentration Checks are broken.

It's possible I do not understand how Concetration is supposed to work since I am attempting to apply logic to it.

Note: this occured in Coelesence Chamber(sp) on normal.