the game seems to calculate the multiplier timer in real-time and not ingame time. what that means is that if your comp is running slow(laggy) then you will lose combos even though you get every gate just right. I really think this should be changed..
try pressing ctrl+"+"(or ctrl+mouse wheel up). it should zoom into your browser so you can see your targets way better. i still only can get about 2k points even with an avg hit of 9.6. dunno how those guys get even more..