Staged Integration Lines and binary conflicts

Hello,

I am using a Staged Integration Lines metodology in SVN where thereis a
trunk (stable revisio) and a branch per Environment (por isntance
Development). When a feature/patch is required then a new branch (featureA)
is created and developers edit files in than branch. When the feature nedds
to be promoted then the changes are commited to featureA-branch and the
changes merged to Development. Finally changes are reintegrated to trunk.

When we only modify text files there is no problem but, if featureA
modifies a binary file, then merge to development for new feature-branches
shows a binary conflict!! Me text files are merged ok.

Test case:
1- Create a trunk with a binary (word file por instance) and a text file.
2.- Create a copy DES-branch from trunk.
3.- Create a copy featureA-branch from trunk.
4.- Edit text and binary in featureA. Commit changes.
5.- Merge featureA to DES.
6.- Reintegrate featureA to trunk.
7.- Create a copy featureB-branch from trunk.
8.- Edit text and binary in featureB. Commit changes.
9.- Merge featureB to DES. --> Shows a binary conflict!!!

Why SVN is not able no guess/track than the binary is also commited to DES?

Thank you in advance

-- 
David Morón Ruano