x := some_code_here says "x is a NEW variable whose type is whatever the result of the expression on the right hand side is".

Say we have a function, GetInt64(), which...returns an int64. x := GetInt64() both declares x to be an int64 and places the resulting value of the right hand side into x. If x was already to declared to be an int64, E.G.:

var x int64

x := GetInt64()

Then the compiler complain that x is not new - it has already been declared. In this case you have to omit the colon to remove the fancy "auto" semantics.

var x int64

x = GetInt64()