Why fps changes in COPY mode?

[admire]

Hi! I open a MTS file (progressive 50 fps) and save it as AVI in COPY mode without any other changes. No changes should occur to the content but after saving into AVI the movie seems to have 100 fps. Why

Thanks for any suggestions!
Marek

[mean]

version ?

[admire]

2.6.7 (64 bit)

[admire]

...on Windows 7