Because they use two different forms of the name. The English chemist Humphry Davy named the metal in the early 1800s, and by 1812 both aluminum and aluminium were in print.
The longer form matches other element names that end in -ium, such as sodium and potassium. British English kept aluminium. American English kept aluminum.
The extra i changes more than the spelling. It adds a syllable and moves the stress: al-yuh-MIN-ee-um against uh-LOO-mih-num. The lu of the British word becomes a light yuh.
Use the form your listeners use, and keep it whole. A mix such as uh-LOO-mih-nee-um is neither the British word nor the American one.