Shouldn't the sum of all net run rates of the teams in a round robin be 0 at any point of time? What could be the reason for a net positive NRR?
It is easy to understand this with some Mathematics. Since this is Sports.SE, I will present an informal Mathematical explanation rather than a rigorous one.
Consider there are 5 teams in the Round Robin group. Then the sum of their net run rates is:
which is equivalent to:
This is clearly not equal to the following expression (which is what your question is probably alluding to):
The above expression, however, will always equal zero, because (a) the total number of runs scored equals the total number of runs conceded, and (b) the total number of balls faced equals the total number of balls bowled. However, that is not what happens when you calculate the sum of net run rates.
In other sports such as football, the sum of goals scored and goals conceded by all the teams in a table will be equal, but that doesn't happen with the net run rates in cricket since there is division involved, and not just simple addition.
For convenient reference, below is an actual points table as an example. It is the points table from Group 2 of the Super 10 stage of ICC World T20 2016 at the end of 25 March 2016.