It's largely marketing; there is a logic to it, but it's a bit tricky. One probe was launched to reach Jupiter, Saturn, Uranus, and Neptune. The other probe was launched to reach Jupiter and Saturn and focus on Titan as it went by Saturn, and preferably to get there first.
Titan was a tantalizing world at the time (still is). It was known to have an atmosphere of methane and be larger than Mercury and the Galilean satellites and was speculated as a place where some form of life might conceivably have formed. In fact, Titan was considered important enough that, if something happened to the probe that was to fly close by it, the second one would sacrifice the trips to Uranus and Neptune for a good look at Titan.
Because of this and the alignment of the planets, the craft going to all four planets had to launch first to arrive at the right time on the right trajectory to reach all four. I think I read once that, to reach Uranus and Neptune, the margin for error at Saturn was very very small, the equivalent of sinking a 900 ft putt without rimming the cup. However, it would also arrive at Jupiter AFTER the Jupiter-Saturn-Titan probe, partly from the math and partly to know which trajectory to take (Uranus-Neptune or Titan). Since the later launching probe was arriving at Jupiter first, it was called 1.
Beer, because my brain hurts