A staffing decision by the Cubs’ stadium operations might have made as big a difference as any “mechanical’’ issue in creating the tarp-related mess Tuesday night that led to the historic decision Wednesday to uphold the San Francisco Giants’ challenge of what was originally ruled a rain-shortened Cubs victory. Sources said the Cubs ordered grounds-crew staffing reductions this week to cover recent “overages” in hours by the crew. The crew’s failure to quickly cover the field with the tarp during a sudden heavy shower in the fifth inning Tuesday night — and spillage while removing it — created unplayable field conditions that could not be resolved during a four-hour, 34-minute delay. The Giants filed a protest. And a swift review by MLB officials resulted in the first successful protest in the majors in 28 years. With Cubs’ baseball officials supporting the Giants’ efforts, MLB determined that the tarp had not been properly rolled up after its previous use, creating a “malfunction of a mechanical field device under control of the home club” — the only provision in the rule book allowing for a “regulation” five-inning game to be suspended.