A fresh post by Matthew Cobb at Jerry Coyne’s blog explains in lay terms why the genetic code is not universal. Keep in mind that the explanation is simplified, and only one presumable mechanism (codon capture) responsible for non-universality is mentioned, while there are a few more distinct mechanisms that have been proposed (e.g., ambiguous intermediate). Also, intriguing to learn that Cobb has just finished the book “Life’s Greatest Secret: The Story of the Race to Crack the Genetic Code”.
Note that what Cobb describes has happened during evolution after the Last Universal Common Ancestor (LUCA – first cells on Earth from which all terrestrial life evolved), so all of that is absolutely compatible with the seeded-Earth hypothesis and with the message in the universal genetic code.