SAN DIEGO — When Jake Arrieta earned his first victory of the season Thursday night in four starts since returning from the disabled list, it underscored one of the darnedest story lines in baseball this year.

The Cubs’ 5-1 victory over the San Diego Padres at Petco Park, behind Arrieta’s six impressive innings, meant that every pitcher who has started a game for the Cubs this season has a victory.

Except the guy with the best earned-run average in the majors, Jeff Samardzija (1.46), whose winless status has reached historic heights.

According to Elias Sports Bureau, Samardzija is the first pitcher in major-league history to have an ERA of 1.50 or better 10 starts into the season and not have a victory.

“I don’t know what to tell you,” he said the day after the bullpen blew a save Wednesday for the third time this season when he was in line for a victory. “I don’t know what to tell you.”

The victory Thursday was the Cubs’ fourth in their last five games, with the five starters producing a combined 1.10 ERA during that turn through the rotation – and only Samardzija among them not coming away with a victory.

“He’s been performing extremely well all season,” manager Rick Renteria said. “It may not carry the win or the loss at the end of the day, but he’s been pitching victoriously, obviously. It’s not taking away anything he’s been doing between the lines every single time he goes out there.”

Meanwhile, Arrieta got the kind of run support and bullpen support Thursday that Samardzija has lacked.

Right-hander Carlos Villanueva followed Arrieta with three scoreless innings of relief to earn a save.

And Anthony Rizzo put the Cubs ahead in the fourth with a two-run homer against his old team and later added a double.

The Cubs tacked on three runs in the fifth on three run-scoring hits.

Email: gwittenmyer@suntimes.com

Twitter: @GDubCub