The Royals have played well this year en route to a 17-9 record. The offense has been gangbusting with the best of them by having a 118 team non-pitcher wRC+. That's second only to the Dodgers, who have a 140 non-pitcher team wRC+. That's utterly ridiculous, especially considering their .298 BABIP compared to the Royals' .331 BABIP (tied for 3rd among teams).
I didn't come here to compare the Royals against the Dodgers though, despite the color similarities. No, I'll compare them to the Brewers, who are more like the Royals in the underlying peripherals department. Spoiler alert: That's NOT a good thing. The Brewers just fired their coach and basically declared to baseball that they have given up on the season by making their veteran players available in trades. The trade deadline is well over two months away.
The Royals are continuing where they left off last year. They aren't walking much (tied with the Brewers for the lowest walk rate), but they also aren't striking out much (lowest strikeout rate in MLB). Lots of contact and BABIP luck; that's when the good Royals showed up last year. The good Royals have shown up this year. The good Royals will go away at some point.
Here's the Brewers comparison.
Data from Fangraphs. PitchF/X plate discipline data.
The basic difference--the Royals make more contact in the zone, which is why the Royals have a much lower strikeout rate. Theoretically, the Royals should make better contact than the Brewers, which they do. The Royals have a higher LD% and a higher BABIP. The Royals actually rank #1 in line drive rate.
However, notice all the similarities. Both teams chase pitches way too much--the Brewers rank the worst in O-swing% while the Royals rank second worst. Both teams are very aggressive--the Brewers have the highest swing rate, and the Royals have the third highest. The zone percentages don't justify the aggressiveness, either. The Brewers rank 20th in zone rate, and the Royals rank 29th. It's not like pitchers are throwing them tons of strikes.
Despite the underlying similarities, the results have been very different. The Brewers wRC+ is terrible, 29th worst in MLB. I have a hard time believing that the Royals' offensive outbursts are sustainable. The line drive rate will come down, the BABIP will come down, and runs will stop crossing the plate at the current rate.
The other side of the regression coin is the pitching. The starters, as a group, haven't been very good. They haven't been the worst. They're somewhere in the middle right now. That's probably where they'll stay. It's the bullpen that seems primed for regression.
Collectively, the Royals bullpen has an ERA. Like, 1.08. It was 1.00 yesterday. The next-closest bullpen is the Cardinals at 1.44. However, the Royals sport a 2.76 FIP and 3.37 xFIP. There are two main reasons the bullpen should regress, and we're familiar with those two reasons here: The BABIP Fairy and the LOB Minotaur.
The Royals bullpen has the lowest allowed BABIP at .191. The Royals have the highest strand rate at 92.7%. It's not particularly close for either stat with regard to who's in second place. The BABIP Fairy normally sprinkles dust. In the Royals' case, the BABIP Fairy acquired a dump truck overflowing with Fairy dust and recklessly rammed it straight into the Royals bullpen. Out of that Fairy dust explosion leaped the LOB Minotaur with its Axe of Stranding, ready to slice and dice opposing lineups as though they were made of additional BABIP Fairy dust, which then also got poured onto the Royals bullpen.
In addition to those things, the Royals bullpen has the fifth lowest homer rate. Residual effects of opponents being turned into dust.
The Royals will always have the defense, though. The defense secretly acquired some BABIP Fairy dust magnets, thus building a supply of Fairy dust. Those same magnets attract the LOB Minotaur for reasons that would make continuing this ridiculous metaphor even more ridiculous. Metaphor over. The conclusion is that it is reasonable to expect the Royals bullpen to maintain a favorable BABIP and/or LOB rate because of the defense and its actual true talent level, which is considerable. Those rates certainly won't be this low forever.
Overall, the offense will get worse, the bullpen will get a little worse (though their actual talent is still extremely high), and the starting pitching will probably stay about the same. Sorry to rain on everyone's Plaza Parade. I'd love to be wrong. Please be wrong. Keep winning. Don't collapse.