Rakshasa and Class Levels

[Constantinos]

I am confused by a rakshasa with ten rogue levels's challenge rating.

Having ten class levels and being a ten hd creature, his challenge rating should be 15 (if rogue is not a key class for him) or 20 (if rogue is a key class). Yes the book and the SRD gives him a challenge rating of 14.

Am I calculating wrong?

And wouldn't rogue be a key class for him as rakshasa use their abilities to stealthily confuse their opponent?

EDIT: I edited out the proper name, as my players read this board.

[Bobson]

CR guidelines are suggestions and approximations, and are always subject to a judgement call.

[Mauril]

Also, books make mistakes.

[Talynonyx]

Consider the amount of gear he has, the situation he is in, maybe he has aging bonuses/penalties?

Edit:

AP Stuff:

Now that I remember who you are most likely talking about, if you read in his tactics portion, he's deliberately holding back so as to not reveal his true identity. And if he finally uses his full abilities, he'll be nearly dead anyway.

Also the AP he is in was published under 3.5 rules.

[Constantinos]

Thanks for reminding me.

I did read that, but I was confused by the SRD translated write up. I had extracted it mentally from the module.