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assignments are also more open-ended... a student may choose to do more research and/or produce a lengthier or more complex product... different levels of abilities are more apparent in the final products
Interesting that this was the description for "differentiated work products".

Differentiation is the term most often used to refer to a teaching strategy, pedagogy, or construct in which there are different expectations, demands, rigor for different students to achieve the same grade. For example, the assignment may be different and/or different grading rubrics may be used. This is typically NOT what gifted students need, want, benefit from in the long term, or advocate for. The extra-work-for-the-same-grade differentiation in task demands often feels punitive and may lead to burn out by providing the "challenge" of futility, causing students to deny their giftedness and hide their potential.

Variability is the term most often used to refer to the range of quality among student work products, when that difference springs from student choice (and not from the dictates of differentiated task demands). For example, students would be given the same assignment and grading rubric. Based on your post, this seems to more closely match what you describe seeing in your child's middle school.