The current metric depends on counting IP addresses within the specified period as a proxy for distinct anonymous editors. A unique IP address doesn't necessarily identify a unique user due to IP rotation, IP addresses shared among multiple editors, proxies etc.
This metric includes edits on existing pages as well as pages that have been or will later be deleted. This allows us to define a metric as stateless, in other words historical values will not change in the future depending on the status of a page (existing/deleted/moved) at the time the metric is computed. Deletion-related activity is tracked via a separate set of metrics.