When the Twins signed Ervin Santana to a four-year, $54 million contract before the 2015 season, it was assumed they were getting a middle-of-the-rotation starter who would give them credible innings — and a lot of them. In the five years before signing with the Twins, Santana averaged 207 innings and a 3.88 ERA. Even though he was 32 when he made his Twins debut, it stood to reason he could at least come close to those numbers and help stabilize the rotation.

His tenure got off to a terrible start, of course, when Santana was suspended for the first half of the 2015 season for a failed drug test. He was decent after coming back that year (7-5, 4.00 ERA in 17 starts). He was much better than his record indicated last season (7-11 with a 3.38 ERA).

Few people, though, would have predicted that 10 starts into 2017, Santana would be pitching more like a Cy Young contending ace than a No. 3 starter. But that's where we stand right now. Santana tossed his second complete game shutout of the season Tuesday against Baltimore, improving to 7-2 with a 1.80 ERA in the process.

How in the world is Santana doing it? Here are five possible explanations:

1) New baseball boss Derek Falvey is known for improving a team's pitching while new catchers Jason Castro (who has caught Santana seven times) and Chris Gimenez (three times) have reputations as good receivers. It's possible their influence has helped Santana.

The holes in that theory, though, are that the Twins' team ERA this season minus Santana's gaudy numbers is 4.72. And Santana was already on an upward trajectory last season before any of those guys showed up, having posted a 2.41 ERA in his final 18 starts of 2016. So maybe those guys have helped a little. But we need to keep looking for better answers.

2) Twins manager Paul Molitor had an interesting quote after Tuesday's gem, complimenting Santana on his ability to vary speeds with his fastball. "He'll paint at 90, 91 [mph], and then elevate at 93, 94. It might not seem that significant, but he knows when he needs to reach back and elevate. He doesn't blow people away, but he gets strikeouts when he puts it in a good spot."

The numbers bear that out, and those subtle dips keep hitters off-balance. According to Brooks Baseball, Santana's average four-seam fastball velocity this season is 93.16 mph, which is right around his career average. But his fastest pitches each game top out around 96 mph.

Then again, he's had that ability throughout much of his career — and even threw harder for his max pitches earlier in his career. So while varying speeds with his fastball is probably part of his effectiveness, it's nothing new. Onward …

3) Santana is getting more movement on his pitches this season than in previous years. Ah, now we're getting somewhere.

The vertical movement on all of Santana's pitches — fastball, sinker, slider and changeup — is more pronounced this season than basically any other point in his career. The biggest differences are with his changeup and slider, which are moving an inch or two more on the vertical plane than they have during Santana's career. That might not sound like much, but even subtle vertical movement makes it harder for hitters to make hard contact.

4) The Twins' defense has been fantastic this season.

I think we have to acknowledge this as a strong factor as well. Santana is pitching great, but he's allowed just 31 hits in 70 innings — which I've triple-checked because it sounds so absurd — without any change to his strikeout rate. Opponents are hitting fewer balls hard and fewer line drives off Santana than they did in past years, but not enough to fully account for the fact that he's allowing just a .134 batting average this year.

The Twins are tied for fewest errors allowed this season in MLB (14) while leading the majors in defensive runs saved (32, almost twice as many as the next-closest team, per FanGraphs). Their defense has been excellent by any measure, and Santana has surely benefited.

5) We're in the dreaded "small sample size" territory.

Yeah, it's dangerous to extrapolate too much from just 10 starts this season. Santana is bound to regress some.

Don't forget, though: Santana's stretch of excellence dates back to the middle of June last year, so we're almost a combined full season of baseball into it.

It's not typical for a 34-year-old pitcher with a track record for being slightly above average to suddenly become great. But maybe the combination of improved defense behind him, better movement on his pitches and veteran guile created the perfect storm for Santana?