Appending to lists and macros do not work together

For the following code:

macro(add_to_array arr1)
    set(cflags ${arr1})
    list(APPEND cflags "myitem")
    message("Resulting list inside macro = ${cflags}")
endmacro()

set(my_array "list1")
list(APPEND my_array "list2")
message("Resulting list before calling the macro = ${my_array}")
add_to_array(${my_array})
message("Resulting list outside of macro = ${my_array}")

CMake prints:

Resulting list before calling the macro = list1;list2
Resulting list inside macro = list1;myitem
Resulting list outside of macro = list1;list2

Why is the update outside of the macro not seen inside? Also if I start modifying the passed variable inside the macro all kinds of very strange things start to happen.