Generic "brain training" apps often measure an abstract internal score that's hard to relate to anything real. Arithmetic is different — accuracy and speed on a sum are about as concrete as a cognitive measurement gets, and they map directly onto something you already do in daily life.
"Brain age 34" or a proprietary index doesn't tell you what to actually do differently. "82% accuracy on division, 94% everywhere else" does — it points at exactly what to practise next.
That specificity is why Math Games reports plain arithmetic stats instead of a synthesised score.
The strongest criticism of brain-training games is that improving at the game doesn't always improve anything outside it. Mental arithmetic mostly sidesteps that problem, because the skill you're training — fast, accurate arithmetic — is the same skill you use splitting a bill or checking a discount.
There's no translation step between the game and real life, because they're the same task.