He has only been in F1 for two years: 2006 and 2007, and 2006 was only part season.
In 2006, Nick Heidfeld scored 23 points in 18 races. That's 1.28 points per race. Meanwhile, Kubica scored 6 points in 5 races, which is 1.2 point per race.
In 2007, Heidfeld scored 61 points in 17 races: 3.59/race. Kubica had 39 in 16 races: 2.44/race. (In comparison with Kovalainen: 1.76/race, Rosberg: 1.18/race, Fisichella: 1.31/race, Wurz: 0.81/race.)
So he's almost matched Heidfeld's performance in 2006, and did a very respectable job in 2007.
It's not that he's a good driver now, but that he, unlike Heidfeld who has been in F1 for 8 years, has plenty of room for improvement. He's still very young, inexperienced, and highly rated by other drivers. Lewis Hamilton was once quoted as saying that he'd pick Kubica as having the greatest chance of being WDC.