The "easy" answer: It's not.  Your method might be just as correct as the teacher's. And this is true at one level - any method that gets you to the correct result repeatably given a wide variety of problems is a correct methodology.  I was just in a brief discussion today, for example, on how the method of long division is taught differently in the US and the UK, for example.  Clearly US students and UK students, given the identical problem, will always get the same answer if they follow their technique correctly.

If this were the only goal - getting the correct result - then what I just wrote could be the entire answer.

But there's more.  The path of more and more advanced techniques is a progressive one where each element of the methodology builds on the previous ones.  If your child misses one of the steps along this path, the later ones will become more difficult.  For example, learning to do division in a particular way might be a necessary step to solving lowest common denominator problems later. Or there might be some part of a technique of doing long division that will be harkened back to when learning about different number bases.

I offer this as "an answer" not necessarily THE answer, since I don't know what your child's teacher is thinking, specifically.

Another possible answer has to do with scalability.  Some methods work well for small problems but fall apart when applied to large problems.  If all you ever had to do was to add numbers together where the sum is ten or less, counting on your fingers would be a valid technique.  But for larger problems it is no good.

The mentality is to keep it one way instead of two ways to avoid confusion. This would be especially true in a traditional classroom setting. However, if you are working one on one with a child or very few children, then there may be time to show them an additional way. I think has to do more with time constraints in the classroom and confusion.

they are doing new programs to teach the kids and the 'old way' (no offence) will just confuse the child.  the problem we have is that they dont teach us how to do it so we cant help the kid but yet they come home with homework.....

i hear your frustration, we have the same problem here.

I went through this with my child too.  At night when she would go to sleep, I would dig her homework out and educate myself.  I wanted to know what the teacher's were teaching my child and how they were doing it.  It helped me to better explain it to her the next time.  I didn't learn half as much when I went to school as she is now!  If I didn't understand her homework, I had no problem going to the school and asking the teacher about it.

Hi Michelle,
There is much debate within the school system about how Math should be taught. Like many other subjects, it can be taught a variety of ways. My suggestion is to try your best to learn how your daughter is being taught in school, and if that works for her. Your knowledge can supplement or be a new approach at any point in her education, and may help to broaden her sense of number concepts as she learns to look at them in multiple perspectives. if she is a struggling Math learner, you may want to keep it simple and stick to what she is learning in school, so different methods don't confuse her.