The AppleDeveloper guide seem to imply that UIAccelerationValue can range between a double value of -1.0 and +1.0.
I have logged the values from a real device whilst "shaking" with crazy gestures my iPod touch and got x values above 2.0 (e.g. +2.1, -2.1) and NO y value above 2.0f.
My take on this is that Apple has implemented some algorithm that approximates the force of gravity and takes as 1.0 values that are above a standard speed approximation (e.g. have values 9.8 m/s of speed).
You may be misunderstanding a part of the documentation. Nowhere does it say that the value ranges between -1.0. and 1.0, as far as I can see. It says that:
The device accelerometer reports values for each axis in units of g-force, where a value of 1.0 represents acceleration of about +1 g along a given axis. When a device is laying still with its back on a horizontal surface, each acceleration event has approximately the following values:
"g" is used in a particular technical sense here; 1 g is one standard gravity; a phone accelerating faster than this will register readings higher than 1. Violent shaking in the hand is easily enough to cause acceleration and deceleration values higher than 9.8m/s2.