What you found is pretty normal in my experience. IE: it’s not broken, there is nothing for you to fix
Why it happens no idea. I could speculate to the cows come home, but it’s all speculation.
Basically sometimes you won’t get 100 channels when using first 100
Example: Browser Categories | Twitch API Example
Here it’s just the game filter. You’ll also get first != record cound when loading with zero filters and just paginating