Because the Netherlands have ODI status.
Let's give them credit though, they have scored almost 250 against a test playing nation. Not that long ago they would have been rolled over for under 200. Plus the 16 year old Kervezee, who will have had nowhere near the amount of training a 16 year old in a test playing country would have had, has fallen just short of a half century. Imagine what he'll be able to do in a few years time.
Give them time, this is only their 14th ODI, and their first in more than three years. They need games against the test playing nations, otherwise, how do you think they'll get better? If they keep ODI status at the 2009 ICC Trophy, and they play Sri Lanka again the next time they tour England, the result could well be different.