(CNN) California is still counting ballots more than three weeks after Election Day.

The slow counting process has been underscored by a series of House races in California carrying on for days past November 6. Now, the only remaining uncalled US House race is in California's 21st Congressional District.

There Republican Rep. David Valadao trails Democratic challenger TJ Cox by 506 votes -- or about half a percentage point -- with votes still being counted. If Cox holds on to win, it would give Democrats a net pickup of 40 House seats this year.

So why is the process so slow?

California officials say the effort is built with a series of safeguards, many of which don't exist in other states. The bottom line: California's vote-counting process is built to take a long time.

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