The ability to specify a Product Attribute Value position by integer was present in Magento 1.x but removed from Magento 2.
Currently, if you want to re-order product attribute values, you need to drag and drop the value. If you're creating a new attribute value on an attribute that already contains dozens or hundreds of values, this means creating the new value at the very bottom of the list, and having to drag and drop it through all of the records above.