Hi,
Do you perform inbox/mobile rendering checks with your templates? If they are more complex, maybe some email providers have more trouble displaying the message in their app, leading to more views in browser.
If you have a way to check this for different email providers like gmail, apple mail, and so on, and for different devices (mobile phones, tablets, computers) as well, that could be a start.
Let me know if you find anything,
Florent