Merging problem

I am new to using git and I encountered the following problem

1) Created a repository using Notepad added a file (FileA.txt). Put one 
line of text in the file. Did a git commit -a. so far so good.

2) I created a branch (my-branch), did a git checkout my-branch

3) Using notepad loaded FileA and changed the first line of text to 
something else. Gave command git commit -a no problems

4) Git checkout master looked at FileA nothing changed

5) Did a git merge my-branch. No conflict reported

6) Loaded FileA in master the text of the first line had changed to what 
is in FileA from the branch

Your help with this problem would be appreciated. I hope this is not due to my lack of understanding

Mike