Comment by Alexander-Barth

I think this is quite similar to julia's handling of a struct. An array of mutable structs is just an array of pointers, where every pointer directs to the underlying structure. However with an array of structs (immutable is the default), there is no such indirection. The value of all fields are stored as array element (unless you have an array of heterogeneous elements).

If you want to change an element of such an array you need to create a new immutable struct which in practice it is quite fast, but a bit verbose to write.