Ok - here we go.
Let`s clear things up first of all.
1 - Forza 2 by default was 270 degrees only and could not use anything else and at the time 900 degree wheels did not even exist for a console.
2 - Forza 3 by default was again 270 degrees by default and designed to run with the Microsoft Wireless Racing wheel at it`s default 270 degrees - but the game was in theory able to accept 900 degree wheels but were glitchly at the time.
3 - Forza 4 by default was 270 degrees again to be matched with the Microsoft wireless racing wheel but when Fanatec came along with the GT2/ Porsche PWTS Wheels - they worked out a deal with Turn 10 to be able to run at 900 degrees for the launch of the game. But when the game demo came out it did not yet work with 900 degree wheels as many found out. They later fixed the problem but the default sensitivity was always 270.
4 - Fanatec was the only wheel that was capable of running at 900 due to there firmware.
In your case I believe you are not configuring your wheel properly to run the true 270 default setting.
First of all if your wheel is connected to the console with any setting other than 270 - and you turn sensitivity to off - you must turn off the wheel power and then power back on and reconnect the wheel to the console. Only after doing this will the game run in it`s true 270 degrees. This is most likely the important step that you are forgetting to do and if you do not do this then the game can not reset from the last known settings..
Turn 10 have stated this
Fanatec have stated this and is in some of there manuals.
Dan Greenwalt has stated this.
The game has complete control over the sensitivity and Fanatec firmware simply overruns the default settings with a - or + gain over the default.
And by saying that you recently started the game and you find that it is at 900 degrees by default ( THAT IS 2.5 TURNS LOCK TO LOCK ) and not at 270 ( 3/4 TURN LOCK TO LOCK ) ?????
Here is a link where Fanatec responds to the sensitivity issue back then.
http://911wheel.de/?q=node/6940