How does the charging cube for the watch differ from all Apple's other ones?

When I bought the watch I used a plug-in cube I had from my iPhone. It didn't charge. Apple explained that I had to use the cube that came with the watch. With a strong magnifying watch I managed to read the gray on gray information on the chargers. Both are identical: .15 amps, 50/60 Hz, and 5 watts. Are you able to charge the watch with any Apple charger? If not, do you understand why not?