What did you purchase?
2N3904 is a JEDEC designation, where the spec includes the package, and limits on the electrical characteristics. Manufacturers usually provide additional data (typical data, tighter ranges, graphs) as well.
If what you got is anything but a normal body TO-92, you didn't buy a 2N3904.
There's no such thing as an "SMT 2N3904" for instance, but there does happen to be the widely available MMBT3904 with identical electrical characteristics. Some other popular numbers may be repeated in this way (MPSH10 / MMBTH10, TIP31C / MJD31C, etc.), but it's not a system, and coincidences abound.
Not that it's a big deal, as there are a hundred kinds of general purpose transistor with largely overlapping specs (from the ancient 2N2222 to '3904, 2N5088, 2SC1815, BC847, etc...), and the main thing you're buying is package and pinout.
Tim