Hi Corrado,
The only thing I can think of that might explain the difference is that in AI Servo mode the camera tries to maintain a lock, whereas in One Shot the lock only has to be achieved once. If the lighting conditions are marginal then it may not be possible to keep the lock in AI mode, but by scanning over the subject you can just about get a lock in One Shot. I haven't noticed this myself, but then again in really dim lighting I tend to resort to flash.
One way to confirm whether this explanation is correct would be to try for a lock in One Shot mode and see how long it takes for the camera to confirm the lock. If you have to hunt around for more than a second or so, then the chances of being able to hold the lock in AI mode are pretty slim.
HTH
-= mike =-