how is game score determined
They are based on your rating and your opponents rating. When you first join you get a provisional rating eg.1200 or whatever you select. It takes some time before this stabilizes and is more accurate and so in your early games you will gain and lose a lot of rating each time you win and lose. Now it is more accurate so you gain/lose much less. If you beat someone rated hundreds of points less than you your rating will only increase a small number or even not at all because your now accurate rating shows that you should beat them 99/100 times or whatever. Vice versa if you beat someone rated hundreds of points higher than you your rating will increase much more because you are expected to win only 1/100. You would even gain several points for a draw vs a much higher rated player because it's a great result for you.
To play only higher rated players I think you would have to set a custom challenge where you set the rating of opponent to between equal your own rating and plus 400. This would mean you would play 900-1300 rated players. I would suggest that 1400 would be too high to play against and the ideal is an opponent only slightly stronger eg.100 above you.