It is the same as:
irb(main):001:0> h = Hash.new(0)
=> {}
irb(main):002:0> h[1]
=> 0
irb(main):003:0> h
=> {}
A default value is just returned if the key does not exist in the hash, not
assigned.
Your example takes the returned value and mutates it, that may mud the
waters, but perhaps the 0 example above clarifies.
Even worse. The *same* array reference is returned for every key miss. The
default value is one particular object, the one you passed to Hash.new.
Therefore, this happens:
irb(main):001:0> h = Hash.new()
=> {}
irb(main):002:0> h[1] << 1
=> [1]
irb(main):003:0> h[2] << 1
=> [1, 1]
irb(main):004:0> h[3] << 1
=> [1, 1, 1]
See, same array reference mutated multiple times (and no entry effectively
created in the hash, still).
The idiomatic way to do this like you expect is John's solution.