The same reason any four key combination can fail on a keyboard that doesn't support more than three keys reliably. Here is an article that explains it much better than I ever could.

Okay. Fair enough Ill take a look at that

Thanks Sid

also on laptop's like mine , the numpad is integrated into the keyboard's alphabet but you have to access it with a function key and the numbad pressed together or a function key plus num lock , to keep it on or off .

extremely difficult because it adds another key or /and effectively disables those alphabetical keys while in use .

From Sid's link: , does this mean that with most keyboards you can actually press 9 keys (6 + 3 modifiers) at the same time, even if it would be a bit weird stance?

Anyway, if I were you, I would check older games to see how they solved the problem of having multiple persons playing on the same keyboard (anyone remember jump & bump with the rabbits? you can play it with 4 persons together on the same keyboard iirc).

Than there are the SNES and Neogeo simulators with e.g. beat-them-ups where 2 players can play on the same keyboard.

Perhaps another possibility is attaching 2 keyboards to the computer (keyboards can be very cheap), maybe that would work even though it would be a bit akward .

even older games had the problem , I remember racing games when played on split screen ,players keys had to be selected carefully because if both players pressed the accelerate buttons while turning , one player couldn't turn unless the other stopped turning.

http://www.microsoft.com/appliedsciences/antighostingexplained.mspx

That's the same article as Sid posted . (which is a good article by the way)

there gaming keyboards with virtually no rollover and ghosting. They can be kind of pricey though. I can confirm i can press 10 keys at the same time with g710+. Did not try with more, i have 10 fingers.

also this article states the 6 key limitation in USB controllers have long been solved:
http://www.tomshardware.com/reviews/mechanical-keyboard-razer-logitech-gigabyte,3505-6.html

last paragraph.

Solution - Contact a Gaming keyboard company and bundle your game with their keyboard like oem MS Windows.

Lol, sorry I couldn't resist.