Weight paint changes after import/export in S4S

Hi !

I would like to change the following weight paint from this :

To this :

But once it is imported and exported in S4S, the normalization is corrupted and I get this :

It seems to happen only with a few combinaisons , like 0.7+0.3, 0.85+0.15.

If you put 0.6+0.4 or 0.8+0.2, the weight paint is not corrupted.

Original book exported from S4S : package

The mesh modified in blender 2.70 (with 0.7+0.3 weights) : blend

The issue happens after import/export.