Passive Polarized Glasses LCDs convert the two frames to the Hortizontial Interleave 3D format because that is the format they use to display 3D:
There are two other 3D formats used besides Frame Packing (only 3D BD uses this). They are called Frame Compatible 3D formats because they use the same bandwidth that HD uses. This results in less than Full HD per eye.
SbS 1080i - each frame is 960x1080i:
Top/Bottom 720P - each frame is 1280x360P
There are two different 3D "formats." The first is what the signal is using and the second is what the 3DTV is using to diplay the left and right images.