According to the White House budget proposal,

"Earmarks Disrupt NASA's Science Activities

Many earmarks in NASA's budget have little to do with the agency's mission in scientific research, technology development, and exploration. For example, the Congress earmarked NASA's current budget to fund corporate jets, college dormitories, libraries, and museums. Some especially damaging earmarks divert funds from critical NASA needs and reverse good cost management decisions at NASA. For example, after costs had doubled,
NASA cancelled its Pluto-Kuiper Express mission last year, but the Congress earmarked funds to put the mission back in NASA's budget. However, the Congress only provided $30 million, while over $400 million more is needed to finish the mission. Congress also redirected $40 million from the Space Station 2002 budget to an unaffordable space test vehicle (X-33?) at a time when NASA is trying to get Station costs under control. Finally, the Congress earmarked funds for a low priority propulsion lab by cutting the very research the lab is meant to support."

The "and subsequent years" comes from the fact that the Congress didn't fully fund PKE last year so additional appropriations in subsequent years would be necessary to build and fly the mission. It's a budgeting thing.

That same reference shows how the number of NASA budget earmarks has grown to over 120 items over the past few years with a total cost of over $500 million out of NASA's roughly $14 billion budget.