The "why" is simple: Different plugs for different amperages. A 30 mp plug/socket isn't the same as a 50 amp plug/socket, and neither one will fit a 100 amp socket.
I can't read the rating on the sockets (not sharp enough focus), and I can't remember the blade configuration to amperage mapping for sure, but I *THINK* the welder plug (and bottom socket) are for a thirty amp 220 circuit. I'm pretty sure the dryer plug should be a fifty amp circuit.
(Or do I have them backwards, guys? Help me out here...)