There's nothing to explain really, the differance really is close in both cases, and then all depend of internal factors, match scenario and other stuffs.
+5 advantadge in some attributes is not, and can not give a definitive result.
And the SA's we talked many times that are not guarantee of anything.
One can not pretend have +1 star in general Q. and assure 3 points.
No.
Not enough.
Case 1, +15 stars with all players, case 2, +16, really the situation is generally+1 and then, count, the game never select all players, there's an scenario, scenario can depend for example of the position of the teams in the classification. Then 1 good player with a good day can kill the game, or test and have a the correct man in fouls/corners can make the differance... and more things..
34 seasons looking matches tells me that if you want a guarantee 90%... maybe the line starts in +3 stars... and we can see more matches and the conclusion probably will be more radical...LOL