Kroger's (NYSE:KR) stock may not be keeping up with Amazon, but the grocer is still beating the market. The stock crushed the S&P 500 over the past year, rising about 32%. This more than doubled the S&P 500's 13% gain during this period. Of course, much of Kroger's recent outperformance can be credited to a big move in the company's stock price last week, when shares jumped about 10% after the company reported its first-quarter results. The surge in the stock price was Kroger's biggest one-day move in nine years. 

With so much of Kroger stock's outperformance tied to last week's jump, investors should take time to understand exactly what drove shares higher. Kroger's first-quarter results offered plenty to like, including better-than-expected sales and earnings per share, impressive growth in digital sales, and more. Here's a closer look at four of the most insightful metrics from the quarter.

A man looking at lettuce as he shops at a grocery store

Image source: Getty Images.

1. Net sales increased 3.4%

For its first quarter of fiscal 2018, Kroger's net sales rose 3.4% year over year to $37.5 billion. This revenue came in about $300 million higher than a consensus analyst estimate for revenue of approximately $37.2 billion. 

Excluding fuel and the impact of the sale of Kroger's convenience store business unit, first-quarter sales increased 2.8% year over year.

2. Same-store sales increased 1.9% year over year

Much of Kroger's first-quarter sales growth was driven by its strong same-store sales growth.

When measured using a method that is comparable to the way Kroger has previously reported the key metric, the company's same-store sales increased 1.4% during the quarter, after excluding fuel sales. But Kroger's new method of reporting same-store sales recorded 1.9% growth. While the new method makes comparisons with Kroger's historical growth in the metric difficult, it is a more accurate way to compare the grocer to its peers, Kroger CFO Michael Schlotman explained in the company's first-quarter earnings call.

When calculating identical sales to be more inclusive of all company business units, including Kroger Specialty Pharmacy and ship to home solutions, our ID sales without fuel were 1.9% in the first quarter. We intend to use this calculation going forward as it presents a comprehensive view of our performance as we redefine the grocery customer experience, and is therefore a more appropriate measure of our performance. We've also looked at what others include in their ID calculations and have taken this step to be more consistent with how our peers report.

If this method is a better way to compare Kroger to Walmart (NYSE:WMT), it puts the grocer on a more even playing field with the supermarket juggernaut's recent performance. Walmart's first-quarter comparable sales increased 2.1% when excluding fuel sales.

3. Adjusted earnings per share increased 26% year over year

Kroger's $0.73 in adjusted first-quarter earnings per share was up 26% year over year -- $0.10 higher than the consensus analyst estimate for the quarter. The strong bottom-line performance "creates a tailwind for the investments we plan for the rest of the year," said Schlotman during the first-quarter earnings call.

In addition to beating analysts' estimates for the quarter, management said its adjusted earnings per share was "a little better" than its internal forecasts.

4. Digital sales soared 66% year over year

Kroger doesn't plan on being left behind when it comes to the growing opportunity to expand sales online. The company's digital sales surged 66% year over year -- and this was on top of Kroger's strong quarter for its online business last year when digital revenue more than doubled. 

This sales growth doubled Walmart's first-quarter e-commerce sales growth of 33%, which was on top of 69% e-commerce sales growth in its year-ago quarter.

Kroger's strong growth in digital sales was driven primarily by its continuing expansion of ClickList (Kroger's online grocery ordering service), delivery initiatives, and same-store sales growth, management said.

If Amazon was supposed to knock Kroger off its game by acquiring Whole Foods last year, it hasn't happened yet. In fact, Kroger's first quarter puts the spotlight on healthy growth and impressive execution online.