I am sure the intern who wrote a rule for the Apple VoiceOver speech synthesizer to special case the number 18 being pronounced english while the synth voice is set to german imagined to have a good reason at the time as well. However, that desn't make ther decision less stupid. "Vierzehn Uhr", "Fünfzehn Uhr", "Sechzehn Uhr", "Siebzehn Uhr", "Eighteen Uhr".