Matt Slocum/Associated Press

Four games into the 2017 World Series, home runs are flying out at historic rates. Pitchers from both the Los Angeles Dodgers and Houston Astros have attributed that to what they're calling "slicker" balls provided by Major League Baseball for the Fall Classic.

"We had a well-pitched game tonight from both sides," Astros pitching coach Brent Strom told Tom Verducci of Sports Illustrated after Game 4. "I just want to know why? Why in the world would the baseballs in the World Series be different? Because you can see the difference. You can feel it. I don't understand it at all."

Dodgers pitching coach Rick Honeycutt said Yu Darvish, who got pelted for four runs in 1.2 innings in his Game 3 start, said the balls hurt his ability to throw sliders.

"Yu noticed the difference," Honeycutt said, per Verducci. "He told me the balls were slicker and he had trouble throwing the slider because of how slick they were. He wasn't able to throw his slider the same way."

MLB senior vice president of baseball operations Peter Woodfork said the World Series balls are made from the same materials used during the regular season. Woodfork said the only difference between the two balls is a gold stamp, which differs from the blue ink used during the regular season.

Strom presented Verducci with two balls, one from the regular season and one from the World Series. The reporter said the two balls "looked and felt noticeably different," and the World Series ball was "slicker to the touch."

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“The World Series ball is slicker. No doubt," Astros pitcher Justin Verlander said. "I’m telling you, we’re in here signing [World Series] balls before the game, and it’s hard to get the ink on the ball sometimes. You know when you sign a receipt at Starbucks, and if you don’t hold the paper down with your hand, the pen just slides across the paper and the ink doesn’t stick to it? That’s what it’s like sometimes trying to sign these balls. That’s how slick the leather is.

“It’s different. I noticed it especially throwing a slider. It didn’t feel the same. The home run I gave up to [Joc] Pederson was a slider.”

Every player interviewed for the piece said they felt a difference between the two balls, and sliders have been disproportionately affected. Verducci noted that Houston closer Ken Giles has missed the strike zone on 75 percent of his sliders in the series.

The 2017 World Series is already tied for the ninth-most home runs in history with 15.