What happens when you use a specific CRF value on a already encoded x264 video? If there is a h.264 video, and I use -crf 17 I sometimes I get a lower bitrate, but the weird thing is - most of the time get a higher ones. 

I am trying to get this setting because I think some videos of mine are having a higher then needed bitrate and I have not figured a way (if it is possible at all) to determine what is the most appropriate way to do this and maybe playing with CRF will not help me at all, but I wondered what actually happens when you don't encode with libx264 for example, but actually use CRF values on a x264 file?
When decoding a file, the decoder attempts to recreate the source material with the information provided in the file. But it can not be a 100 percent accurate representation, so the decoder creates new pixels that are estimates of what was included in the source 

When you encode for the second time with CRF 17, the encoder does not know these new pixels were generated by the decoder and were not necessarily faithful to the original material. But the encoder still does its job, and attempts to encode these new values accurately. 

So it spends bits encoding pixels that were thrown away and recreated by the first encode, making the file larger.