The difference is in the license that you accept before downloading the specification. I'm surprised you didn't notice this as you carefully studied each document! For the JSRs I've checked, the documents are identical—including the implementation license in-lined in the document.
The evaluation link offers a "Limited Evaluation License" for evaluating the specification. I think this is aimed at JCP participants, public commentators, and application developers that want to understand the specification.
The implementation link offers a license to implementers. They get a royalty-free license to distribute implementations under certain conditions. Paraphrasing loosely, these requirements include: that the implementation completely implements the specification, that the implementation doesn't modify the java
package namespace beyond what the specification allows, and that the implementation passes the applicable TCK.