iMac wifi MUCH slower than Mac Air?

I bought the latest iMac with 32GB RAM, the i7 chip and the 512GB SSD hard disk. Top of the line. US$ 4,500.

I also bought the Mac Air with 8GB RAM, i7 chip and the 512GB SSD. About US$ 2,500.

Both within January 2014.

I have fiber optic connection at home. With the best ASUS router. When I take the Mac Air next to the router, speedtest.net shows me I'm downloading at 275 mbps, uploading at 135 mbps.

When I take the Mac Air into my room, which is sadly separated by two walls, the Mac Air speedtest goes down to download of 50mbps.

Kept right next to the Mac Air is the iMac. Speedtest on iMac, at the very same location, shows 3mbps.

Why's this? How can a much expensive and powerful machine clock 3mbps, less than 1/10th the wifi speed of an Air? Is the wifi card or whatever much lower in quality?

What am I missing? Would appreciate any pointers.

Thanks