Through analysis of a classroom lesson led by a decorated teacher, we illustrate how instructional practices favor students seeking empirical patterns at the expense of using mechanistic reasoning. In the lesson, when students spontaneously come up with hypothetical mechanisms to explain why a light bulb in an electric circuit does or does not light, the teacher, following the guidance of standardized curricula, redirects them toward pattern-seeking. We argue that this bias toward pattern-seeking in Chinese national standards and curricula, along with ambiguity in those documents on what an "explanation" is, sits in tension with students' productive abilities and propensities for engaging in mechanistic explanation. In discussion, we extend the argument to show a less severe but similar bias toward pattern-seeking in the United States' Next Generation Science Standards.