In fact, arduino doesn't sense values from 0 to +5v on analog pins. Instead, it sense from 0 to +4.5v which is exactly 10% less.
Uh...no, that's not quite true. The Arduino senses analog voltages relative to its supply voltage (Vcc) which depends upon where it's coming from. With external power, this voltage will be 5V +/-4% (generally...this is what linear regulators commonly provide). Now, if the Arduino is USB-powered, then Vcc is whatever is coming out of your computer's USB port (and perhaps past hubs and such), which can be quite a bit lower than 5V (I believe the spec at the PC itself is at least 4.4V).
So you are either doing some math wrong, or you are using a USB-powered Arduino with a "low" USB voltage.
--
The Basic Motor Driver: simple, inexpensive motor driver for 1 stepper motor or 2 DC motors