60.119 FPS rather than 60 - what am I doing wrong?

17:43:01.776: ===== Starting mpv ======
17:43:01.777: Command line: G:\SVP 4\mpv64\mpv.exe I:/Backup/H/Phoenix Point mod materials/fmv/test/JP/rexlogo/4K/REXLOGO.webm -o I:/Backup/H/Phoenix Point mod materials/fmv/test/JP/rexlogo/4K/REXLOGO.SVP.temporary.mkv --no-audio --no-sub --no-sub-auto --input-ipc-server=mpvencodepipe --input-media-keys=no --no-msg-color --vf=vapoursynth:[C:\Users\dgali\AppData\Roaming\SVP4\scripts\ffff.py]:4:16 --of=matroska --ovc=libvpx-vp9 --ovcopts=lossless=1,row-mt,quality=best,threads=16
17:43:01.819: (+) Video --vid=1 (*) (vp9 4096x3072 15.000fps)
17:43:01.819: Audio --aid=1 (*) (vorbis 2ch 18900Hz)
17:43:02.161: [autoconvert] Converting gbrp -> yuv444p
17:43:07.061: VO: [lavc] 4096x3072 yuv420p
17:43:07.061: [vo/lavc] AVOption 'row-mt,quality' not found.
17:43:07.061: [vo/lavc] Opening encoder: libvpx VP9 [libvpx-vp9]
17:43:07.387: [encode] Opening output file: I:/Backup/H/Phoenix Point mod materials/fmv/test/JP/rexlogo/4K/REXLOGO.SVP.temporary.mkv
17:43:07.388: [encode] Opening muxer: Matroska [matroska]
17:43:07.717: V: 00:00:00 / 00:00:15 (0%) {0.0min 0.0MB}
17:44:07.759: V: 00:00:03 / 00:00:15 (21%) {2.2min 2.7fps 428.0MB}
17:45:08.475: V: 00:00:04 / 00:00:15 (32%) {3.0min 2.2fps 867.6MB}
17:46:08.678: V: 00:00:06 / 00:00:15 (44%) {2.3min 2.1fps 1069.9MB}
17:47:09.294: V: 00:00:08 / 00:00:15 (59%) {1.0min 2.1fps 1056.4MB}
17:48:09.464: V: 00:00:10 / 00:00:15 (73%) {0.4min 2.1fps 1183.0MB}
17:49:04.673: [encode] video: encoded 1325638687 bytes
17:49:04.674: [encode] audio: encoded 0 bytes
17:49:04.674: [encode] muxing overhead 11647 bytes
17:49:04.759: Exiting... (End of file)
17:49:04.836: ===== Starting mkvmerge ======
17:49:04.836: Command line: G:\SVP 4\extensions\code\mkvmerge.exe -o I:/Backup/H/Phoenix Point mod materials/fmv/test/JP/rexlogo/4K/REXLOGO.SVP.webm I:/Backup/H/Phoenix Point mod materials/fmv/test/JP/rexlogo/4K/REXLOGO.SVP.temporary.mkv -D I:/Backup/H/Phoenix Point mod materials/fmv/test/JP/rexlogo/4K/REXLOGO.webm
17:49:04.955: mkvmerge v35.0.0 ('All The Love In The World') 32-bit
17:49:04.955: Automatically enabling WebM compliance mode due to destination file name extension.
17:49:04.995: 'I:/Backup/H/Phoenix Point mod materials/fmv/test/JP/rexlogo/4K/REXLOGO.SVP.temporary.mkv': Using the demultiplexer for the format 'Matroska'.
17:49:05.053: 'I:/Backup/H/Phoenix Point mod materials/fmv/test/JP/rexlogo/4K/REXLOGO.webm': Using the demultiplexer for the format 'Matroska'.
17:49:05.053: 'I:/Backup/H/Phoenix Point mod materials/fmv/test/JP/rexlogo/4K/REXLOGO.SVP.temporary.mkv' track 0: Using the output module for the format 'VP8/VP9'.
17:49:05.053: 'I:/Backup/H/Phoenix Point mod materials/fmv/test/JP/rexlogo/4K/REXLOGO.webm' track 1: Using the output module for the format 'Vorbis'.
17:49:05.054: The file 'I:/Backup/H/Phoenix Point mod materials/fmv/test/JP/rexlogo/4K/REXLOGO.SVP.webm' has been opened for writing.
17:49:05.114: Progress: 0%
17:49:05.616: Progress: 4%
17:49:06.261: Progress: 8%
17:49:06.761: Progress: 12%
17:49:07.443: Progress: 16%
17:49:08.075: Progress: 21%
17:49:08.720: Progress: 25%
17:49:09.376: Progress: 30%
17:49:10.016: Progress: 34%
17:49:10.650: Progress: 39%
17:49:11.273: Progress: 44%
17:49:11.915: Progress: 49%
17:49:12.567: Progress: 53%
17:49:13.228: Progress: 58%
17:49:13.860: Progress: 63%
17:49:14.510: Progress: 68%
17:49:15.187: Progress: 72%
17:49:15.815: Progress: 77%
17:49:16.464: Progress: 82%
17:49:16.966: Progress: 87%
17:49:17.557: Progress: 90%
17:49:18.106: Progress: 93%
17:49:18.739: Progress: 98%
17:49:18.974: Progress: 100%
17:49:18.974: The cue entries (the index) are being written...
17:49:19.106: Multiplexing took 14 seconds.

I believe it's actually 60 fps, but mpv/ffmpeg always (?) encodes as "variable frame rate" so "60.119" is an estimated value.
If you know ffmpeg option to make the output "constant frame rate" - let us know