Users behave differently in different scenarios. There are so many factors involved that you can't make definitive conclusions from what they do at the surface level. It's probably better to look at what's behind user actions.
Same situation, different behaviours
It happens to me, and it might've happened to you too; in a situation that was literally the same, you behaved differently based on that moment's context. We humans are excited one day, and dull the other. Sometimes we're with a girlfriend, other times with dad. Human behaviour is so fluid, you might struggle to repeat your own behaviour from one day to the next.
What does it mean for designers? Is user research kaput? Certainly not. but you might want to look through user actions and arrive at the whys.
Users say one thing & do another
Some people buy an SUV because they think they'll go offroading. For others, it's the thing everyone's doing these days, so they like to fit in. Some might like the extra space, still others might prefer it for transporting sports gear. Whatever the whys are, all of these people go ahead and perform the same action: buy an SUV.
I am pretty sure car companies know all of these whys, and then some more. SUVs in general are quite spacious, but most are not designed for offroading. Didn't the SUV designers know some people might want to go offroading? Surely they do. But they also know that good intentions don't always result in actions. So plenty of people think they'd like to go offroading, but they never actually do. May be the car designers know this difference between what people say and what they actually do.
The key thing here is to understand what percentage of people have that particular why. If it's an edge-case, designers can safely focus on other more important things.
The Hawthorne effect in user research
There's a recognised human bias that occurs when people behave differently because they know they are being watched. It's called the Hawthorne effect. I suspect Hawthorne effect comes into play whenever we carry out moderated usability testing or discovery research. People are self-conscious and want to appear smart. Their behaviour with your service at home might be vastly different.
The 'why' behind user actions
We humans tend to be reasonable beings most times. I say reasonable because usually whatever we do, we have a reason to do it. That reason is the why behind our actions. But our reasons are not always logical or obvious. Is there a deadline looming? We tend to work faster. But if there's something else at play such as a date with a girlfriend, the deadline can wait. We have our reasons for everything we do, although the reasoning could be unreasonable by various standards.
Sometimes it's just a matter of asking. During one usability testing session I was facilitating, the participant kept going back and forth between pages of a form he was supposed to fill out. But after a couple of attempts, he was able to complete the process. My initial hunch was he was just checking if he's filled out the form correctly. Just to be sure, at the end of the test, I asked him why he was navigating back and forth. His answer? He thought he was at the wrong place because he'd filled out a very similar-looking form in the previous test. He just wasn't sure if he was at the right place.
Now this confusion caused by similar-looking forms was the real insight I needed. Another user might completely ignore this confusion, even if they did find it confusing, possibly due to the Hawthorne effect.
Is user research repeatable?
User researchers can make their process repeatable, but users are fickle. They'd do one thing the first time, and do another next. There's no way you'll be able to get exactly the same reactions, feelings, behaviours and comments from a user twice. So, the only way to find real insights is to dig deeper and find the why.
Are the whys repeatable?
Whys can be valid, but more often than not, they're not consistent with each other. They are just as varied as humans are, with no unifying theme. There could be just more reasons for users to do what they do. Once you corroborate the discoveries of why in your research, you'll understand the various motivations, mindsets and preferences of those user cohorts.
By the way, if you've read this far, you should let me know your why on Twitter.