I was reading the Asian Wall Street Journal (WSJ) and came across an ad for a property in the Carribean. It said, "An 1,100 hectare..."
"An" is used when the next word starts with a vowel, so technically, this is correct. Although "one" starts with an "o" it is pronounced with a consonant (i.e. "w").
Try reading it out loud, "an 1,100 hectare..."
Weird, isn't it?