Why are boards replaced in a laminate floor duller than the others?
Interesting question. It sounds like the installed flooring has become polished by wear or was at one time sealed with a gloss finish.

If you have additional replacement material you can test various finishes to match the installed ones. Laminate manufacturers recommend sealing the flooring after installation primarily to protect the joints from spills and from absorbing water from humid air but also to make the floor easier to clean.

Try the manufacturer's recommended sealer first, then matte, semi-gloss and gloss, then compare to the installed sections in various lighting scenarios.

If you determine a suitable match, mask the original planks, apply and rejoice.