Can anyone explain to me the logic of "concurrent sentences"...

[Pert_UK]

Now OK, I can understand it when somebody gets convicted of 5 counts of stealing from various shops, and the judge orders 1 year jail for each count, to run concurrently.

To me, that is indicating that that type of stealing merits a 1 year sentence - to spend one year in jail for each count (i.e. 5 years) would be over the top in this instance.

I can also (kind of) understand it when, say, a serial sex attacker gets given a 15 year sentence for each attack, to run concurrently - I presume that this is so that each individual victim can see that their attacker has been given a sentence for the attack on them.

What I can't understand is cases like this:

http://story.news.yahoo.com/news?tmpl=story&cid=816&ncid=816&e=2&u=/ap/20040326/ap_on_fe_st/bribe_money

Essentially, a guy got sent to prison for sexually assaulting a child. He then arranged for his wife to bribe a prosecutor into arguing for his early release.

"Joseph Messier, 64, pleaded guilty to bribery in December 1998 and was sentenced to four years in prison, concurrent with his sex assault sentence. "

Essentially, this guy has been given no extra jail time for attempting to bribe his way out of jail, even though this was a separate crime of bribery meriting a 4 year sentence.

What is the philosophy behind this? Can anyone explain?

(To be clear here, I am not singling out the US, the same thing happens in the UK and is equally puzzling. Also, can we leave out the appropriateness of the specific sentences cited - personally I would be quite happy for serial sex attackers to be locked away for ever, but that isn't part of the debate on concurrent vs consecutive sentencing as far as I can tell).