I'll preface this by agreeing that we will see where we are later in the season. But, I'm going to offer some reactions to the opinion that things like OPS+ are more predictive of what's going to happen in the future than looking at runs scored in the past.
First, the records against teams over .500 (which at this stage of the season are the same as the teams that I am considering playoff contenders) does refute the reaction that the Brewers can't beat good teams. Though that record got a recent boost with the games against the Jays and Rays because before winning the last 4 of those games the Brewers were 15-21 against contending teams. I think those games have had a reassuring effect.
That aside, one of the things I have been watching is not just the Brewers wins and losses against contending teams, but what the offense has done against them. The Journal Sentinel had an article about this about two weeks ago and highlighted how much more productive the Brewers hitting has been against weaker teams.
I described in a post a couple of weeks ago that I am tracking how many runs the Brewers have scored against contending and noncontending teams. This is very easy to do using data in the Schedule and Results section of Baseball Reference.
I have now updated those numbers for games through yesterday (and including today's Brewers and Cardinals games). They continue to confirm that the Brewers run production is dramatically higher against noncontending teams. In the 40 games against noncontenders, the Brewers are averaging 5.88 runs per game, but in the 40 games against contending teams they are averaging 3.43 runs per game. That's a pretty significant difference of almost 2 1/2 runs per game.
When I first did this I wondered if this kind of discrepancy was typical. At least for the teams I checked (Cardinals, Dodgers, and Giants) it is not. The Cardinals numbers are 5.19 vs non contenders and 4.44 vs contenders, a difference of 0.75 runs. For the Dodgers those numbers are 5.54 amd 4.54 (a difference of 1 run). I haven't updated the Giants numbers but when I looked at them a month ago they were actually scoring slightly more against better teams.
So. the question or concern that I continue to have is whether the Brewers offensive numbers have been propped up by the high number of games against the noncontending teams that allow the most runs (Reds, Nats, Pirates, Cubs) and that they will have far fewer games against those teams in the second half than they have had in the first half. As I mentioned in an earlier post, by the end of next weekend the Brewers will have played 41 of their 87 games against those 4 teams. They will only have 22 games against those teams in the second half plus 12 against the Rockies and Dbacks (who are not quite as generous as those bottom NL Central teams).
You made the statement that things like OPS+ are more predictive of future scoring than past runs scored. I asked the question yesterday, and didn't receive an answer, whether a stat like OPS+ includes a factor based on competition. But, if it doesn't, wouldn't it be fair to expect that as competition gets tougher, the OPS+ of the players and the run production for the team might come down too? I understand that we can't arbitrarily remove the extraordinarily high scoring games like the 18 and 19 run games the Brewers have had against the Reds and Pirates. But, for predictive purposes I question how much value those games have toward how a team might be expected to perform in a playoff setting against the best pitchers on the best teams.
We're not going to resolve this now, but it will be interesting to follow going forward.