A "static" table size is only interpreted by the browser as something like a strong suggestion. But the number cannot be held to rigidly by the browser's rendering engine, or else authors could create valid code that is, nevertheless, impossible. That would crash the browser.
So the way out for any browser's internal code is that if the content needs to push the static table beyond its stated dimensions, then it's allowed to.
You probably need to re-think the template - or perhaps place stricter controls on what content can be placed inside your inner, relatively sized table.