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.
A couple more points to add to Kappy's post:the modem decodes the incoming signal and passes that to the router to transmit to the connected devices. The router then keeps organized all of the devices on the network, wired and wireless. The radio signal that the router sends out is independent of the receiving device, but receiving devices, your MacBook and iPad, may show different signal strength due to their individual antennas. You cannot do anything about the individual devices unless one is not working right and you want to have it tested.
Before doing that you might want to do some troubleshooting: http://support.apple.com/kb/TS1398 and see if anything there helps.