Factual error: Jack Sparrow says he loves the French because "they invented mayonnaise." The film is set in the 1750s. The egg-based sauce known as mayonnaise was developed in the early 1800s. The Oxford English Dictionary says the word dates from 1815.
Factual error: The governor of the island claimed the bank safe (roughly 2 meters square on a side and with walls about 13 cm thick) weighed "an imperial ton." A safe of this size would weigh nearly 22.5 imperial tons.
Factual error: The British Ship of the Line is flying a version of the British Union Jack that was not adopted until 1801. The story is supposed to take place between 1720 and 1750.
Answer: There was never any explanation other than Elizabeth kept it in the chest. As both the Elizabeth and Will characters are rumored to return in POTC 6, the question may be answered in that installment.
raywest ★