This is inspired by the (closed) Bug 29220: In analogy with the map type rules, it could make sense to explain typing of arrays in more detail. The following paragraph or a similar one could be added to 2.5.5.9 of the XPath and XQuery 3.1 specs (...provided that it's correct):

"The function signature of the array, treated as a function, is always function(xs:integer) as item()*, regardless of the actual member types in the array. This means that a function item type with a more specific return type, such as function(xs:integer) as xs:integer, does not match an array in the sense required to satisfy the instance of operator. However, the rules for function coercion mean that any array can be supplied as a value in a context where such a type is the required type, and a type error will only occur if an actual call on the array (treated as a function) returns a value that is not an instance of the required return type."