English is a living language. Living languages change. Words take on new meanings or fall into disuse. Modern translations use contemporary English to relay the meaning of the original Greek and Hebrew. Moreover, no translation is perfect.
Translators are human.
All translations contain errors. The King James translators were human. They made errors just like other translators before and after. They would be the first to admit the need for periodic revision and correction over time. Still, the KJV has much to recommend it.