Browser zoom in/out optimization vs. responsive design

I have a situation where we have a website that is designed to be responsive (tablet, phone). Our business stakeholder discovered an issue when the site was zoomed out to 90%. His argument is that a responsive site should, by definition, respond at different zoom levels. I know this is not the case - site zooming and responsive design are not the same thing. But I do not know how to adequately explain this. Could someone help me? Thanks!