I tried using one Dlink modem and one innatech modem on 8mbps, but the dlink always maxed at at speeds of 6.5mbps on speedtest while the innatech reaches 8.7mbps.
On the Dlink is set ADSL2+ Annex L and Annex M enabled, wifi uses Wireless N at Auto Bandwidth speed .

What could be the reason Dlink cant achieve the speed of 8mbps?

The test consistently shows Dlink only can achieve 6.4-6.6mbps while Innatech can reach 8.7mbps.

I managed to fixed it. after reset the modem it can get 8 mbps.