Wall Street's hitting all-time highs, and it's naturally a team effort. More than 750 exchange-listed stocks achieved fresh highs last week, but they don't all necessarily deserve their good fortune. 

Universal Display (NASDAQ:OLED), American Express (NYSE:AXP), and SeaWorld Entertainment (NYSE:SEAS) were at new highs last week. Let's go over why these recent market winners are surprising names taking victory laps.

Two American Express cards, in gold and rose gold.

Image source: American Express.

Universal Display

The allure of OLED -- or organic-light emitting diode -- technology for panel displays is crystal clear. Products leaning on Universal Display's OLED provide crisp displays that take it easy on power requirements. Universal Display has been a rocky stock. It rose when it was incorporated in the high-end iPhone model, only to sputter when sales of iOS smartphones languished.

The key to Universal Display's turnaround is that OLED is also making waves in TVs, tablets, and even automotive dashboards. There are opportunities in larger screens than smartphones, and naturally bigger prices to pay. Revenue may have plummeted 27% for all of 2018, but it's clearly on the comeback trail these days. 

Reported revenue more than doubled in its latest quarter. A shift in accounting standards helped boost top-line results, but revenue still would've risen a hearty 49% under the old bean-counting methodology.  

American Express

The old "don't leave home without it" ads that applied to American Express credit cards might also apply to the stock. American Express has been hitting new highs over the past few weeks. The financial services giant is paying off for investors despite delivering a rough first quarter in April that featured a decline in earnings and a weaker-than-expected 7% increase in revenue.

American Express could be falling prey to its own confidence. It's been updating some of its signature card lines, jacking up annual fees in the process. Investors naturally love the implied pricing elasticity that when a company can afford to hike its prices, but there are rumblings within the Amex-owning community that American Express may be going too far.

Starting next month, its association with Priority Pass -- a lounge-access perk -- will nix meal credits at participating airport restaurants, eliminating a benefit that is still going to be available through rival credit card providers. American Express is also making it harder for owners of its higher-end cards to use airline incidental fee credits. Unlike its rivals that offer broader travel-related rebates on their signature cards, American Express is clamping down on the loophole where its incidental fee credit could be redeemed through the purchase of select airline gift cards. American Express won't miss the active plastic churners who aren't helping its bottom line, but shareholders may not see it the same way once accounts growth takes a hit.

SeaWorld Entertainment

One of the more surprising soaring stocks is trading at levels last seen six summers ago. SeaWorld stock is riding a big wave despite posting its weakest revenue growth in more than a year last time out. There's also been some heavy promotional activity within the theme park industry this summer, a sign that the peak travel season may not be as hot as the temperature readings.  

SeaWorld has the right approach in getting over the Blackfish stigma that has dogged the brand since 2013, emphasizing new rides and foodie festivals over its flagship marine life attractions. Sector consolidation is also in the air, a tide that is lifting all ships. SeaWorld still has a lot to prove after its blowout 2018. 

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.