As far as I know, there are 4 ways to declare a dictionary in Swift:
var dict1: Dictionary = [:]
var dict2 = Dictionary
All you're doing is noticing that you can:
Use explicit variable typing, or let Swift infer the type of the variable based on the value assigned to it.
Use the formal specified generic struct notation Dictionary, or use the built-in "syntactic sugar" for describing a dictionary type [String:Double].
Two times two is four.
And then there are in fact some possibilities you've omitted; for example, you could say
var dict5 : [String:Double] = [String:Double]()
And of course in real life you are liable to do none of these things, but just assign an actual dictionary to your variable:
var dict6 = ["howdy":1.0]