metronomes (or "pendula") when on table, oscillate with random phases, since that is how they started and they are "uncoupled" (no energy/information flows from one to other so they do not "know" each other.) When they are all together on the cans, notice that the cans themselves oscillate little, providing coupling/information crossover. which forces "synchronization" in periodic systems.
a compelling illustration why multitrack recording is counter-productive to the production of musical energy? while editing a digital recording of a big-band concert, I noticed the 5 leading reeds were perfectly aligned, their triangle waves all in synchronous peak; when co-located in close proximity (relative to the listener) synchronous waves would give a power function growing exponentially with the number of players.