I though when someone got positive feedback from DT member they must attain trust score as well,atleast one.
Where? You must be talking about the risked amount. That's not necessary and only means something to the score when it increases (with time).
Example:
- If you want to make a rating stronger, increase "Risked BTC". 50 extra risked BTC is equivalent to an additional rating.
For the score number (the first 0):
The first number is the user's trust score calculated based on how consistently they've received positive feedback. Probably no one will get a score above 0 until the system has been around for at least a month. The second number is the number of reported scams. The third number increases with the number of positive reports, as does the fourth number in parenthesis, though the fourth number is more resistant to abuse. This text changes color depending on the score. Users with a negative score (attainable through scamming) get a red warning attached to their posts.
It's not only possible to have a score of 0 for a few days after the trust was placed, but when the system was implemented, everybody had a score of 0 because it requires time to increase.