Wifi not getting ISP max speed...works for lan-wifi transfer

I am using a D-Link DIR-815 Wireless N 300 Dual Band Wireless Router.

I am using a Dell laptop for some testing. It supports the 2.4/5ghz dual band.

When using 5ghz right next to the router I get the full 30-35 Mbit connection(speedtest.net) which is what I pay for through the ISP(Charter). The issue is that even right next to the router I can't get the full data transfer at 2.4ghz, it is approximately 20Mbit. I was intrigued and assumed that interference from other devices in my apartment building were causing interference and droping the speed of my wireless connection below that of my ISP.

I decided to buy Lan Speed Test(totusoft), and Lan Speed Test Server. This tests the network speed between two pcs by transferring data from PC1 RAM to PC2 RAM and measuring the time. I ecpected this test would yield the same result and be limited to 20MBit. I was surprised to find that transferring from wired PC1 to wireless PC2 I was getting results of 40Mbit ul /40Mbit dl.

So, my question is how can my wireless signal be capable of transferring 40Mbit up/down to a lan PC but I cannot get the full 30Mbit through the ISP.


Also, before anyone asks why I care since I am getting full speed on 5ghz, the 5ghz only works in very close proximity to the router. I am trying to get higher speeds out of the 2.4ghz band for a PC located farther away from the router.

Cheers
-D