undesrtanding get_post_meta function

get_post_meta() calls get_metadata(), the parameter description of which says:

$single
(bool) (Optional) If true, return only the first value of the specified meta_key. This parameter has no effect if meta_key is not specified.

Default value: false

So, since you are passing '' as the meta_key, the $single parameter is ignored.

Does that explain it?

Edit:

If you only want the 1st value for each meta key, then just do the following:

array_map ('array_shift', get_post_meta (get_the_ID (), '')) ;