The RO2 meta score is based on significantly more reviews than RO1. While both scores are individually curious, they are useless for comparison.
The RO1 reviews are 2006 era reviews, a time of WWII spam, in which Red Orchestra was a unique and refreshing commercial release. I think this enticed tired reviewers and earned it some extra loving on the final score.
2011 hasn't been oversaturated with WWII titles, or even multiplayer shooters besides Call of Duty for that matter, so RO2 is not the surprising indie breakout title that needs a helping hand from reviewers to get recognised. In fact, RO2 was fairly well marketed and does not need anyone's pity or help to prove itself. The review scores are therefore more straight faced.
Honestly, this year has seen a lot of changes in professional gaming reviews, there is way more spread in scores, very positive reviews rarely make it far past 85/100, and only exceptional game of the year type of titles make it past 90/100 these days. I doubt we will be seeing many Half-Life 2 kind of reviews ever again ("98/100, this is the best game EVER!"), where basically anything worse than 9 is a mediocre game, and 7 is something to avoid.
75/100 is a realistic score for RO2 (though for me personally it should be somewhere in the mid 80's, a great 2011 game). 81/100 is close to how I would rate RO1 today, but accounting for review score inflation in the past 5 years I think RO1 should be scoring between 70-75.