How much of a difference are you talking about?
What type of devices? iphone 4 vs ipad Air? Or are they all new devices? I get the same speeds on my 5s, ipad air and iMac via wifi.
5, air and windows laptop.
My iPhone 5 was stuck at 0.5 Mbps on WiFi until I updated from iOS 6 to iOS 7. After the update, WiFi speeds were comparable to my Windows laptop.
Yes it's normal because your laptop has much more processing power and probably 3 antennas. I have a 120 mbps connection, wifi about 90 mbps on the laptops, and about 35 mbps on an iphone 5 and a 4S. We also have a 3GS that does about 16.
Just to make sure.. you are not performing the test on all three devices at the same time, aren't you?