Matroska format's framerate is not accurate with certain values
MKV does not store a frame rate in its container. So ffmpeg/vlc looks at the default duration of a frame instead. It sees 33.333ms and does the math, which is where your weird number comes from. 

For more information, see: https://gitlab.com/mbunkus/mkvtoolnix/wikis/Wrong-frame-rate-displayed

To test this out, you can run ffprobe with -show_frames. I think you'll find the presentation times for each frame are spot on.