Although there has been some sporadic discussion about adopting a standard, the community has never actually ruled on the number of spaces which should follow a period or other full stop. Until a definitive ruling comes from forum discussion, editors are at liberty to use either one space after a period — which is the current standard on most online style sheets — or two, which was the prevailing rule in business until about the year 2000.
That said, a code cleaning bot does actually ensure that only one space exists between a full stop and the next character. However, this bot is not in continuous operation, which means that you will occasionally find instances of double-spacing around the wiki.
If you don't understand the underlying issue, you might want to read more at Wikipedia's in-depth article about sentence spacing.