Referral bonuses are a customer acquisition lever, so brands tune them based on what they're trying to achieve. Some reward only the new signup, treating the offer as a pure acquisition discount. Others reward both sides, turning existing users into an active referral channel.
Bonus size also shifts by region and season — a brand pushing for growth in a new city will often run a richer offer there than in a market where it's already established. None of this means codes are unreliable; it just means the 'best' code today may not be the best next month.