Why don't these two skills combine?

The Bazel Helm Beta helmet provides the Earplugs Level 2 skill and allows a level 3 and level 1 decoration slot. Equipping an Earplug Jewel 3 into the level 3 slot does not give the expected result of earplugs level 5. Instead, on the equipment summary screen, it shows earplugs level 3, not 5.

For comparison, I purchased a Bone Mail Beta, which has Attack Boost level 1 and an open jewel slot. Placing an Attack Jewel 1 into the open slot results in a combined attack boost level 2.

Why might the combining of skills work for Attack Boost, but not for Earplugs? Is there a subtle prerequisite I am missing here, or is this perhaps a bug?

The Earplug Jewel 3 only provides a single level of the Earplugs skill, not three. The "3" in its name only refers to the slots needed to socket it, not how many skill points it provides.