tmpgenc x264 compared to ffmpeg x264

[secondplace]

i've been playing around with tmpgenc and i noticed a significant difference between my ffmpeg files and the tmpgenc files in how the GOP's are filled.



i've attached a screenshot to show the difference. the top is ffmpeg and the bottom is tmpgenc.
i also tested this with x264 and results are similar to ffmpeg and far away from tmpgenc.

video is 1920x1080px, 25fps
ffmpeg x264 settings are the following: preset 'very slow', refs 5, keyint 100, keyintmin 51, bitrate 3500k, maxrate 10000k, bufsize 1120k
and VBR 2pass encoding.

i also matched all the ffmpeg x264 settings to the tmpgenc settings. those where: refs 4, subme 9, chroma_qp_offset -4, lookahead_threads 1, bframes 2, b_pyramid 0, weightp 0, nal_hrd vbr
i left 'slices' out since it has to do with the image being split up not the GOP's

i also exported the quantization matrix from tmpgenc and tested it with x264 and ffmpeg but it didn't help either.
by changing the custom quantization matrix in tmpenc the files still turn out like in the screenshot.

i also tested every single setting in the tmpgenc advanced tab to see if i can basically break the output and make it look like ffmpeg but that did not work either.

how does tmpgenc control the GOP's to achieve that result and not have the frames peak as it happens in ffmpeg. it feels like the bits are distributed way more even within the GOP's and not with the high peaks as it happens in ffmpeg. also tmpgenc really seems to consider the maxrate where as ffmpeg seems to ignore it.
also if i split the maxrate and buffer in half to turn the average bitrate into a (fake) max bitrate in ffmpeg it does not give me those results.

how much does the peaking matter in terms of decoding and streaming the videos? is it irrelevant as long the GOP's are are the same size!? my CPU usage at least is identical by playing the files.

does tmpgenc have a custom x264 build to achieve this?