It's not a guaranteed extra possession; more like an extra half of a possession. Think about it. There are two ways a half can go.
1) Team A gets the first possession, Team B gets the last possession
2) Team A gets the first possession, Team A gets the last possession
Whoever wins the tip is Team A. There is a 50% chance that they will get one more possession than their opponent (if scenario #2 above comes to pass) assuming that there is enough variation over 20 minutes of basketball to prevent the opening tip from affecting which team will have the last possession.
Well if it is an extra half possession then the adv is less then 1%. A 50% chance of getting the extra poss means the adv an extra poss gives is half.