I think the difference is that a proposal is "here's how I would implement your requirements as documented" while an evaluation is a bit more than that. It's "Here's what problems you are having. Here's what you say you need. Here's what we think you need." and maybe "Here are a few ways we could go about helping you."

The goal is not to just deliver a proposal based on an incomplete spec, but rather to help the customer understand their problems better.