When watching a 1080p stream on a 1080p display using Twitch in Google Chrome with the default view layout and the favorites bar visible of Chrome, the video is actually downscaled from 1920×1080 to 1340×751 — not 1340×754.
This seemingly small difference makes the stream noticeably blurrier due to less optimal downscaling.

By hiding Chrome’s favorites bar, the stream displays at 1340×754, which results in a much sharper image thanks to a more precise scaling ratio.

On Firefox, because the favorites bar and top menu take up less vertical space, the stream is displayed at 1340×754 by default, resulting in better image quality without adjustments.

I can provide screenshot examples showing how 1340×754 looks significantly sharper and clearer than 1340×751.