There are also bandwidth limitation on the server side, not all servers connected to the internet are on multi gigabit connections. For a speed test server to benchmark/measure speed of close to a gigabit, it needs AT MINIMUM more than a gigabit. That Maxis server maxes out at around 300Mbps like your results has shown.
The DiGi server seems to have a bigger pipe, and the test is a valid as it has indeed sent 800megabits per second down that fiber cable into your house, demonstrating that your device installed/configured IS capable of receiving the subscribed package's d/l speeds.

eg. in the case of Google Fiber a couple years back, it was know that you cant always achieve the full gigabit speed on the open internet because the (website) servers weren't fast enough. Similar scenario here, just that its happening on the Maxis's Speedtest server.

Easily enough, you can look for 1gig+ speed test servers on [ http://www.dslreports.com/speedtest ] (configure for them in the test settings) OR [ http://speed.googlefiber.net/ ] to see if you can exceed that 300Mbps speed on Maxis's servers.

PS: All the talk eg. bad mouthing isp saying "its rigged", "ok during initial stage", "below par speed (30%) often", "speed restarts at higher level after router reboot", suggestion to "put a timer to rest the router start afresh often" is hearsay at best, unsubstantiated/uncorroborated, old/outdated advice that is often repeated by the ill-informed.

it is just that, sometimes in Malaysia we have first class facilities but third class user mentalities. that's why we get complains like these all the time, everything is 'bad'.