As was reported, the Steam Deck OLED comes with SteamOS 3.5 preinstalled, while the stable channel is still on 3.4.
This means if reviewers didn’t take care to properly match the software versions on the OLED and LCD models, they might not compare apples to apples. The effect is probably small, but so is the reported performance and power draw difference between the models. Also SteamOS 3.5 introduces the saturation slider which in its default setting tries to emulate sRGB more, and could affect screen comparisons.
See for example Taki Udon’s first video, where clearly it is visible that MangoHUD is an older version on the LCD model https://youtu.be/3C6ekDyO18s?t=807
So unless it is explicitly mentioned or shown otherwise, assume that part of the differences might be due to different software versions.
The MangoHUD version shown in the performance overlay clearly is different in the screenshot (Gundam LCD is v0.6.9-1-22-g1d8f9f6 and OLED is v0.7.0-54g9393066). So possibly one is on 3.5 and one on 3.5.1 which is not as bad as 3.4 vs 3.5, but still not properly matched.