XL2 30p footage exported to flash - aspect ratio
 

We have some footage that was imported from a Canon XL2 it was 30p and by accident the whole project and everything was set to interlaced. When we went to export to the adobe media encoder we chose flash and selected preserve aspect ration. It resized accordingly and produced a skewed clip (fat). We manually overrided the settings and did a 4:3 400x300 and it looked fine.

Why is this?