The strength of the signal is dependent upon the device that is actually transmitting. Wireless is two-way so all devices are either sending or receiving. The device's effective signal strength also depends upon the antenna.
Wireless devices are usually tested in special ways to set the transmitting power to the level required by the FCC. This is done in a free-space environment. Once in use any sort of obstruction between devices can attenuate the signal reducing the effective range of use.
If you think your device is not performing as it should then take it to your local Apple retailer for help.