I may have missed something Grant, but I could not understand in your workings how you came to this exact answer.
Why can it not be a score like 1575 and 1725 ?
I assume I am missing something but could not find it..
Not sure how you worked out your numbers. I'll give you some practical examples using the round 1 games:
Simplest way to write the formula is:
Expected teams score = teams average - opposition teams average + 1650
Sydney = 1708.5 - 1448.5 + 1650 = 1910
v GWS = 1448.5 - 1708.5 + 1650 = 1390
If you used a ratio then GWS would score higher than their average because the sum of the two teams average <3300 and obviously GWS shouldn't be expected to score more points against a good team. Note that in the two matches between these sides last season Sydney averaged 1893.5 and GWS 1401 (only 6589pts were scored).
Ade = 1663.3 - 1724.1 + 1650 = 1589
v Gee = 1724.1 - 1663.3 + 1650 = 1711
Geelong would be expected to score 13pts less because Adelaide are 13pts better than average. Last season Adelaide's win helped them score 1739-1556.
Mel = 1447.4 - 1603.3 + 1650 = 1494
v StK = 1603.3 - 1447.4 + 1650 = 1806
Because Melbourne score 202.6pts less than average St.Kilda would be expected to have a big score even though they aren't a great team themselves. Real score from last season was 1556 to 1743.
Haw = 1742.1 - 1590.4 + 1650 = 1802
v BL = 1590.4 - 1742.1 + 1650 = 1498
Real score from last season was 1806 to 1492.
So back to the original question:
1600 ave team = 1600 - 1700 + 1650 = 1550
v 1700 ave team = 1700 - 1600 + 1650 = 1750
Hope you understand it now. I'm not sure I can explain it any better.