Why do people refer to Maryland as a commonwealth instead of a state?

Question

Why do people refer to Maryland as a commonwealth instead of a state?

Answer 1

If they do, they are in error.  Maryland has at no time been a commonwealth.  See section on history at site below.

Answer 2

I didn't know they did - Massacheussetts, Kentucky, Virginia and Pennsylvania are all officially known as "the commonwealth of...." rather than "the state of......", but Maryland should be a state.  Not that it makes any difference anyway - all four commonwealths have exactly the same type of constitution as the other 46 states - it was just a nice way of them saying "We're not dictatorships" when they were set up.