How keyword checks work
Every keyword gets a fixed minute of the day, derived from its id, so a check happens at the same time each day and a day-on-day comparison compares the same moment. Checks are spread across the whole 24 hours rather than run in a burst at midnight.
Every result is validated before a position is stored: the number of organic results has to match the depth, a known domain for the country has to be present, and the top 20 must not have churned more than a configured share against the previous check. A result that fails is quarantined, retried up to three times through the next provider, and never stored as a position. A day with no validated check is a gap on the graph, not a zero.