When calculating psnr with my own tools (python/numpy) I get identical
results compared to ffmpeg and tiny_ssim if I look at the values per frame.
However, if I run ffmpeg || tiny_ssim an I look at the calculated average
I get something different. This I don't get!

Per frame data:
ycbcr.py	ffmpeg	tiny_ssim
26.194		26.19	26.194
25.950652	25.95	25.951
25.73425	25.73	25.734
26.296376	26.3	26.296
26.298215	26.3	26.298
26.324584	26.32	26.325
26.671997	26.67	26.672
26.365405	26.37	26.365
26.250325	26.25	26.25
26.915763	26.92	26.916
26.747261	26.75	26.747
26.774038	26.77	26.774
27.126952	27.13	27.127
27.597209	27.6	27.597
27.794023	27.79	27.794
28.070067	28.07	28.07
27.949201	27.95	27.949
27.996252	28		27.996
28.066139	28.07	28.066
28.084215	28.08	28.084
27.970032	27.97	27.97
27.958326	27.96	27.958
28.030713	28.03	28.031
28.190591	28.19	28.191
28.079677	28.08	28.08
28.285431	28.29	28.285
28.113496	28.11	28.113
28.342119	28.34	28.342
28.502148	28.5	28.502
28.268838	28.27	28.269
28.391678	28.39	28.392
28.417471	28.42	28.417

My calculated average
27.429920125	27.43	27.42984375

When running tiny_ssim:
Total 32 frames | PSNR Y:27.340  U:37.820  V:40.238  All:28.950 | SSIM Y:0.83673 U:0.92946 V:0.95301 All:0.87157 (8.91322)

when running ffmpeg:
[Parsed_psnr_1 @ 0x2ee3960] PSNR y:27.339519 u:37.820406 v:40.238118 average:28.916038 min:27.338981 max:30.077372

tiny_ssim and ffmpeg give same result, which is different from the average calculated above.