I read the article. Note that, after reading the article:
- We don't know what they assessed
- We don't know how they assessed it
- We don't know anything about the comparison groups, including what they are comparing the treatment group to.
- We don't know anything about what the app does. What subject matter? How is it represented? What do students do with it?
- Their definition of rigorous is "demands a score of 10 out of 10." That's rigor? What's it have to do with learning?
- How did they research their treatment? Where did they publish?
If I had to guess wildly, they're giving students some sort of repeated practice in some algorithms procedures or math facts, and then assessing those facts and procedures directly. If this is the case, their results would not be too surprising. But that's a wild guess on very little information. Looking into it a little further, the apps seem to focus on counting to 10, counting to 20, and the times tables for 2s, 5s, and 10s. That sounds like something fairly reasonable to teach to someone in 6 weeks.
Without actual research, we're left to speculate as I have done above. You can take my comments above as addressing your "what could be the causes" question.
In short, the causes could be: 1) comparison to a situation that was previously lacking in time actually spent on learning. 2) Math content that relies on memorization and repetition rather than understanding. We already know that certain things, when drilled repeatedly, can result in gains performing tasks or recalling facts. If they're essentially creating a mechanized and more engaging form of drilling, and testing exactly those things, then you will see gains.