Changing MySQL auto_increment_offset
You find the explanation of your question in the docs: "If the value of auto_increment_offset is greater than that of auto_increment_increment, the value of auto_increment_offset is ignored."

For your purpose it would be sufficient to leave auto_increment_increment and auto_increment_offset on their default values and simply run:
ALTER TABLE test AUTO_INCREMENT=30000 on the slave.

This will result in values like 30001,30002,...,N on your slave.