Entropy comes from the random selection process and list size, not from how unusual the finished phrase looks.
Add another random word for a high-value or offline-guessable secret; each word from a 7,776-item list adds about 12.9 bits.
Keep every passphrase unique and use multi-factor authentication; strength cannot contain the damage from reuse.
The full explanation
Composition rules — one capital, one digit, one symbol — measure the wrong thing and push people toward predictable patterns. What resists guessing is entropy: how many equally likely possibilities the generator could have produced. Modern guidance has moved to length and blocklists, and away from the rules almost every site still enforces. Read the complete guide for the reasoning, examples and definitions behind this answer.