Each deal is scored from the Duplicate Bridge Scoring Table (Law 77), which sets trick scores, premiums and penalties; Declarer's side scores if it makes the contract; the defenders score if it fails; Vulnerability, shown on the board, raises both the bonuses and the penalties
Source: Law 77, Law 78 — American Contract Bridge League — Laws of Duplicate Bridge
Why people argue about this
People often think that if a deal is vulnerable, only the bonuses are doubled for both sides, forgetting about the penalties which also double, leading to potential score discrepancies. The confusion arises because the rules explicitly state Vulnerability affects both bonuses and penalties equally on either side of the table, but many overlook this detail in their calculations.
