It just so happens that the building where I work has no street address, as it is part of a university campus. When I asked Siri where I was, it gave me an address, which I put as my work address in my contact card. However, whenever I asked Siri to give me directions to work, it would lead me astray – about a mile down the road. Yet when I double-checked by asking Siri where I was, it aid I was at the address previously reported. I also discovered that if I manually touch the address in my contact card and touch Directions to here I would get directed to the correct spot! I played around with Siri today and figured out a work-around so that I could ask Siri for directions to work and get there properly.

I believe this is all due to some sort of Apple Maps bug. Manually touching to get directions versus asking Siri for directions to the same address should give you directions to the same place. I suspected that Siri was using different address mappings than Maps was …

Source: Mac OSX Hints