Explicitly Typecasting Numbers into float64 for ExpressionFunctions

[DanSeb1295]

functions := map[string]govaluate.ExpressionFunction {
		"strlen": func(args ...interface{}) (interface{}, error) {
			length := len(args[0].(string))
			return (float64)(length), nil
		},
	}

	expString := "strlen('someReallyLongInputString') <= 16"
	expression, _ := govaluate.NewEvaluableExpressionWithFunctions(expString, functions)

	result, _ := expression.Evaluate(nil)
	// result is now "false", the boolean value

why does the non-functional expression version handle the type-casting into a number type, while the functional version needs us to explicitly typecast our returned value into a float64? it fails and gives me a comparator/modifier error when i remove the explicit float64() typecasting.

callbackA := func(args ...interface{}) (interface{}, error) {
	return 123, nil
}

callbackB := func(args ...interface{}) (interface{}, error) {
	return 123, nil
}

callbacks := map[string]govaluate.ExpressionFunction{
	"callbackA": callbackA,
	"callbackB": callbackB,
}

expression, _ := govaluate.NewEvaluableExpressionWithFunctions("callbackA() >= callbackB()", callbacks)
res, err := expression.Evaluate(nil)
fmt.Println(res, err)

Returns:
Value '123' cannot be used with the comparator '>=', it is not a number