Anhur

Win Rate
The statistical chance of winning a match when playing as the selected god in the selected game mode, with a custom build. For builds, the Win Rate is the statistical chance of winning when buying the displayed items in the designated order.
Margin of Error
The margin in which the actual win rate of the god is, with a 95% certainty. A smaller Margin of Error means a more accurate win rate. Items with a Margin of Error above 5% are excluded.
Sample Matches
The sample size of the data, which is the amount of found matches with the selected god and game mode combination. A higher sample size improves the quality of the data, and data is derived from matches found over several days.
Popularity
The percentage of matches ending with the given build, of matches where the god had 6 non-starting items.

Empty Builds
This is caused by a lack of data. Without a substaintial amount of data, it can be impossible to produce quality statistics, and the app shows nothing in place of builds which are lacking data, as these would be bad representatives of the actual best- and most popular builds. It's better to show no data, than faulty data.
Acknowledgments

The win rate of an item can become skewed, if favored by either players of above or below average skill level, e.g. an item favored by above average players will do better in the statistics, because its win rate is influenced by the average success of its major audience. Matches where the god is building towards the Auto Items build are excluded, in order to block the influence this could have on items in these builds.

Items bought late in the game have a tendency to have a slightly higher win rate than is representative for those items. This is due to you getting ahead of your opponent is making you increasingly more likely to win, while falling behind is making you increasingly more likely to lose. Meaning you are more likely to win a match where you end with a full build, than you are a match where you have not yet bought items for say slot 5 and 6.