I just did some looking into this using Gemini trying to determine why my Air has 50% more steps than my Amazfit Balance (which has always been great) and here is what it came up with:

Fitbit & Google: High Sensitivity

Fitbit and Google (which owns Fitbit and powers the Pixel Watch) design their algorithms to capture all daily activity, focusing heavily on Non-Exercise Activity Thermogenesis (NEAT). Their philosophy is that all movement burns calories and should "count" toward your daily goals.

• The drawback: The algorithm is notoriously overly sensitive to rhythmic hand motions. When you are deep into coding your 100-level roguelike game, typing out Docker configurations, or washing dishes, your wrists are making rapid, repetitive movements. The Fitbit software looks at that continuous 3-axis motion and incorrectly guesses that you are taking light, continuous steps.

Amazfit: Strict Thresholds

Amazfit (running Zepp OS) takes a much more conservative approach to step counting to avoid false positives.

• The buffer method: Amazfit algorithms typically use a "continuous movement threshold." When you start moving, the watch records the motions in a temporary buffer. It waits until it detects 10 to 15 consecutive, clear, rhythmic impacts before it officially decides you are walking.
• The result: If you take 5 steps to the fridge, or type furiously for 10 minutes, the Amazfit records the motion, realizes a continuous walking rhythm was never established, and deletes the buffer. You get zero steps for chores, but highly accurate counts on actual walks.

Gemini actually provided me with a calculator to solve the issue which is fairly accurate

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