GetTopNRankings will give you the top N scores and it is possible that one user can fall in the list multiple times. But GetTopNRankers wii give you the top N Rankers(unique users). For example: If there are three users who has played the game and have scored as follows:
1. A has scored 250,240 and 100
2. B has scored 150, 125 and 50
3. C has scored 175, 140 and 25
Now, GetTopNRankings with N = 3 will give you 250(A), 240(A), 175(C), whereas GetTopNRankers for N=3 will give you 250(A), 175(C), and 150(B)
Coming to your second question, the scores come from the server in the sorted order(default is Descending Order). So the rank is (i+1) where i is the index you have used above.
Let me know if you have any further queries.