I have seen that but if you really make pipe threads up "wrench tight" they will pretty much seal. That is particularly true if there is some lube in there and with that oil they put on pipe to keep the factory cut threads from rusting (or the cutting oil itself when field cutting) they can get "gas" tight, particularly over time when you get a little oxidation going.
The air system I put in my garage (galvanized, schedule 40, threaded pipe) had a very slow leak for a while. Now it holds 150 PSI for weeks with the compressor off.
