Vladimir Guerrero's Data

I am starting off with something simple that involves one of my favorite players of all time, Vladimir Guerrero, Vladdy was known for his aggressive hitting approach which he would consistently swing at pitches out of the strike zone. He had the skill to still drive the baseball even though the pitch was considered a ball. The whole point of this analysis was to see if Vladdy's BB/SO ratio was pretty consistent throughout his career, but looking the first graph below we can see he walked more than he struck out during his "prime years" (which obviously makes sense). Afterwards I was curious if the reason he had a better BB/SO ratio during those prime years was because he was producing more home runs. Looking at the third graph below we can see that statement is not necessarily true. For example, comparing his 2002 season vs. his 2004 season Vladdy's numbers in terms of at-bats (614, 612), hits (206, 206), and home runs (39, 39) were near exact to one another. What may be the reason that his BB/SO ratio dropped by .5? I would think the reasoning was he adjusting from National League (Montreal) to American League (Angels) pitching. All the data was gathered from the Lahman baseball database through RStudio and the only difference between the second and third graph is I added the years to the data points.