I hope I understand what you´re trying to ask. Say the X axis is not continous but discrete as in a sampled system, and you have point A,B,C that make interval A1 and A2.
Point A is for example at sample number 0, point B a 13 and point C at 21. I would say that A1 is then 13-0=13 samples long, and A2 is 21-13=8 samples long.
I am not including the last sample in the measurement of the lenght, because then you get into trouble when you want to add the lengths together. If you did count the last sample, then when adding the lengths of n intervals you would have to subtract n-1 to get a consistent and correct result, which seems unnecessary and overcomplicated.

Is this what you are trying to discuss?

 

So the answer is A1 + A2 = 13 + 8 = 21
which is the same as 21 - 0 = 21
which can be determined by ignoring B altogether.
In other words, there is no need to know what occurs in between.

 

I finally understand what you are asking, after all the discussions. The problem you are having is because you are regarding each of the points A and B as containing a quantity in the horizontal axis. They do not. They are not samples. They are points between selected samples and each has a singe relative value on the horizontal axis. The quantity B-A only includes the total of the samples between the two points.
I hope this answers your original question.