golang中这段代码在print时，为何会输出Error()方法的执行结果

package main

import (
    "fmt"
    "strconv"
)

//ErrNegativeSqrt is alias of float64
type ErrNegativeSqrt float64

//String return the value of string
func (e ErrNegativeSqrt) String() string {
    return strconv.FormatFloat(float64(e), 'g', 10, 64) + " is the result"
}

func (e ErrNegativeSqrt) Error() string {
    return fmt.Sprint(float64(e), " occurs the error")
}

func main() {
    var a ErrNegativeSqrt = 2
    fmt.Println(a)
}

为什么这段代码会输出Error()方法的结果？当把Error()方法删除后，就会输出String()方法的结果？