First, you'll notice that the odds are best in the Thirties but that the expected values were highest in the last decade or so. That's because of all of the extra teams, and therefore games, that are playing today. More games mean more possibilities to hit for the cycle.
Also, you'll note that the actual cycle totals more closely match 5-plate-appearance expectations that 4-PA or the Avg. expected. There could be a few reasons for this. First that the players who hit for the cycle have better odds to hit a single, double, triple, or home run. Second that the odds to get a hit increase after a player gets a hit, i.e., the theory of the hot bat. Third that players who hit for the cycle do so in high-scoring and therefore high-plate-appearance games, thereby bettering their odds. And lastly, players that are hitting for the cycle are able to better their odds by focusing on their goals.
An interesting study could be conducted by studying the batting records for players who are one hit away from the cycle. Do they raise their own odds? Or is it just the luck that comes from playing in a high-scoring game?
My personal opinion is that there is an element of luck but that players can be streaky and can help themselves focus and achieve a goal such as hitting for the cycle. The higher-than-expected actual totals throughout history indicate to me that something more than dumb luck and a couple extra at-bats in an odd game are at work.