What does "doesn't add up" mean?

[Rebecca L.]

So, why does "doesn''t add up" mean doesn''t make sense in the sentence "I can't understand why she did that, it really doesn't add up"?
Can "add up" mean make sense directly? Please help me out......

[Conchita]

Welcome to English-Test, Rebecca!

'Not add up' is an idiomatic phrase which isn't normally used in the positive form.

[Jamie (K)]

"These numbers don't add up," means that when I add these numbers together, the answer comes out wrong.

If a report in the newspaper says, "57% of the population wants a change in the healthcare system, 48% like their current healthcare, and 25% have no opinion," the numbers don't add up, because instead of coming out to 100%, they come to 130. This means there is something suspicious about the study.

This arithmetic meaning of "doesn't add up" has been extended to various kinds of situations. If all the clues in a situation don't come together in a way that makes sense, we say that things "don't add up".

[Rebecca L.]

Thank you very much for your answers.
You remained me that numbers don't add up are what exactly don't make sense!