That’s not always the case. If a house has a well and later gets water from a utility, they will often keep exterior taps running well water because it’s a lot cheaper than abandoning the well. So, technically, you could have water that’s safe to drink inside the house but still have unsafe water outside.

Also, if the house has filters or other water treatment that generally isn’t used for the exterior (though that’s typically more about taste and mineral content, rather than anything hazardous).