Luke Cage's skin is as tough as titanium, but titanium is not bulletproof so how is Luke Cage bulletproof if his skin is only as hard as titanium?
@pokeysteve: That's different because people don't say that Superman's skin is as hard a steel they just call him the man of steel. And people don't say that Ironman's suit is as hard a Iron that's just his name.
It's exactly the same. Is he really a "Man of Steel"? Is Iron Man's armor really made of Iron? Of course not. Cage's skin is not "as hard as titanium" but tougher. It happens all the time in comics. Except Iron Man's armor probably does have some traces of iron in it.
Titanium is bullet proof too depending on thickness, bullet caliber and other variables.