Why conflict when merge a branch that the directory is unmodified

Hi,

Suppose I have two branches with following commit histories:

B----C---D(directory abc/ modified)               Branch1
    /
L--M---O(directory abc/ not modified)            Branch2

In previous graph, the Branch1 merge Branch2 in commit C. After commit C, the Branch1 modified some contents in director abc/. Branch2 doesn't modify directory abc/ since commit M, which has been merged into Branch1.

Then, when I do:

  git checkout Branch1
  git merge Branch2

the director abc/ contains conflicts, why? Because the director abc/ doesn't changed since commit M in Branch2, so directory abc/ should not have conflicts.

Thanks.