Double render length bug?

My project was at 60fps, i rendered via the moviefileout top and selected 60fps - fine. when i changed it to 30fps and changed the FPs in the blue box bottom left, my rendered movie doubles in length, im sure im missing something obvious but why is this?

Thanks

Is there a reason you aren’t just setting it in the MovieFileOutTOP? Most developers hardly touch anything in the timeline.

Solved. Thanks for your reply, i think it was just simple maths i had mistaken.