The general rule in the trade is, tubing is specified by OD, and pipe is specified by ID. So 3/8 copper tubing will have an OD of 3/8", but 3/8 pipe will have a larger OD. Also, tubing will be closer to the nominal dimension than pipe will.
The key is that if you buy tubing, buy tube fittings. If you buy pipe, buy pipe fittings.