Sam West had a nice career playing for the Washington Senators and St. Louis Browns from the late 1920s to the early 1940s.
Check out his 1935 and 1936 seasons, which perhaps look identical:
1935: 527 AB .388 OBP 4 triples 10 homers 70 RBI
1936: 533 AB .386 OBP 4 triples 7 homers 70 RBI
Pretty much identical, right?
Here’s another illustration of why a lot of raw numbers can be deceptive, especially without context.
In 1935, West played in 138 games, getting 615 plate appearances. He walked 75 times (12.2% of the time) and struck out 46 times (7.5%).
In 1936, West played in 152 games, getting 637 plate appearances. He walked 90 times (14.8% of the time) and struck out 70 time (11.0%).
So, he bumped his walk rate up a little, but his K rate shot up by more than 40%.
His homers dropped from 10 to 7, by itself not all that meaningful. But, he also hit 11 fewer doubles (37 to 26) and lost 61 points off his slugging percentage (.442 to .381)
He also lost 22 points off his batting average (.300 to .278). All together, despite getting more plate appearances in 1936, his total bases dropped from 233 to 203.
To make matters worse, league-wide run scoring was way up in 1936, at 5.19 runs per game, from just 4.90 runs per game in 1935.
Factoring in all of these differences, and it turns out that West’s OPS+ was 111 in 1935 but just 88 in 1936. He went from having a good above-average season to a pretty disappointing below-average one. For 2011 comps, his 1935 was similar to what Yunel Escobar and Alberto Callaspo did while his 1936 was similar to what Austin Jackson and Martin Prado did.
But, you say, he still drove in 70 runs each year, so what does it matter? Well, we don’t have his detailed batting splits but I’m guessing he hit a lot worse with runners on in 1936 than in 1935. We can see that in the earlier year, he batted a lot more in the 1 or 2 position in the lineup, whereas in 1935 he got a lot more time at the 5 and 6 holes. Almost certainly he had more RBI opportunities in 1936 and yet batted in just the same number of runners.