1. The Input data could be scraped incorrectly (all input data is available and can be checked under the game in the detailed games page, and goals data is available on the goals page).
2. Outliers – we have tested the calculator as well as we could, but a Belgium vs San Marino can always throw up an outlier that is so heavily biased towards the favourite that the splits are not representative. Over time we hope to identify these issues and correct them, but be extra careful with games with a large favourite or underdog.