GoPro why high frame rate does not reduce time On latest GoPro 4 Silver with 64 GB

when i set 1080p 30fps i get estimated 4 hour recording time but when i double the framerate it not only does not shoren it actually increases so with 1080p 60fps it shows ~4.5 hour.

How is that?
The GoPro4 Silver model uses the same bitrate for 1080p-30 as 1080p-60 and hence the recording times are similar. This does mean that fewer bits are allocated per frame in the 60fps mode, although since the codec H264 uses inter-frame compression, the difference won't be as stark.