Confused by Process module design for the pid's type

All following 3 methods return Int64

def : [Int64]
def self.ppid : [Int64]
def self.pgid : [Int64]

But, all methods accept a pid args as Int, and Int is limited.

Error: can’t use Int as the type of an instance variable yet, use a more specific type

Why not just all use Int64?

Never mind, it just working.