My x264 encodes with same settings keep outputting at different filesizes

I'm using MeGUI 2913 on Windows 10. I've got 2 1080p 30fps video clips which are the same format as each other. For some reason the "Raw" clip when encoded with a particlar CRF setting multiple times will have a different output filesize each time but the other clip has the same output filesize each time. For the clip that has the problem here are some example results I get:

CRF 20 Slow: 91.4 MB, 96.5 MB, 103 MB
CRF 21 Slow: 103 MB, 91.4 MB, 49.4 MB

As you can see the above results make no sense. In some cases the CRF 21 output clip has a larger filesize than CRF 20. In another case the CRF 21 clip had 50% lower filesize than CRF 20 which also doesn't make sense.

I've also just done a CRF 20 and CRF 21 encoding of the "Raw" full length episode. Here's the result:
CRF 20: 5.68 GB, 6.4 Mbps
CRF 21: 8.05 GB, 9.14 Mbps

Again this is wrong as the CRF 21 has a much bigger filesize than the CRF 20 video when in fact it snould have a slightly lower filesize than the CRF 20 video.

I've downloaded the latest updates via the MeGUI program for x264 and everything else and I'm using the development version of MeGUI.

For my test clip sources I find that it's easiest to replicate the problem by encoding the source clips to CRF 20 then encode the source clips to CRF 21. Then repeat until you have output files of a different size. I do my encodes in OneClick. Source clips and log are here:
https://drive.google.com/drive/u/0/folders/1vGpduKF-S9lF2n9uH6_aRcg9ihXVvaP6