The shapes of a stream of water exiting a nozzle range from a compact water stream (also referred to as a solid or straight stream) to a fog stream where the stream is dispersed into small droplets.  I suspect the stream exiting as an elongated bulb is a combination of both. The nozzle begins to disperse the water droplets into a fog stream, but the electrostatic-cohesive property of water molecules attracts the water back into the solid stream, creating the bulb shape.