Warum sollte es (in PHP) anders sein?
Weil die Schlüssel eben nicht ["1"], ["2"], etc sind, sondern [1], [2], etc, also vielleicht irgendeine fucking Bedeutung haben könnten.
"An array in PHP is actually an ordered map. A map is a type that associates values to keys. This type is optimized for several different uses; it can be treated as an array, list (vector), hash table (an implementation of a map), dictionary, collection, stack, queue, and probably more. As array values can be other arrays, trees and multidimensional arrays are also possible."
Quelle: php.net