Hi Elinor,
I am not sure why you expect them to be of exactly the same size. The TTatlas was not made to match TT_N27. The TTatlas is based on the contours drawn in the Talairach and Tournoux atlas. Other atlases such as the TT_N227_CA* were warped to match the TT_N27 dataset and do have the same grid (same number of voxels in all directions).
However, this does not mean that the TTatlas does not apply to TT_N27, nor does it mean that TT_N27_CA atlases were created using the TT_N27 brain (I know of at least one person that would object vigorously to that).
cheers,
ziad