XG is a total evaluating the quantity and quality of chances. So if you have a 100 shots per game from half, it'll add up to pretty much 0.01xG as they're all chances that have pretty much 0xG. A penalty is worth ~ 0.75xG, as roughly 75% of penalties are scored. So basically, the best strikers are the ones who find themselves in quality chances consistently. Not about being clinical off of half chances. All the best goalscorers just keep getting big chances because your movement and ability to get into the positions is pretty much the key attribute, and then as long as you are an average finisher (1 goal for 1 xG over time), you'll rack up the goals. Cavani is generally slightly below, but he's at 96% of his total since 2014/15 which is pretty much there.
Of course, the larger the sample size, the more accurate the info. If you play 1 game and it was a sub and you took a penalty in that time, your season total will be pretty much 0.75xG for the minutes played normalized over 90 minutes (so it'll be something stupidly high). That's why usually it's not that useful to look at below probably 750-1000 minutes, as the variance is too high.