Why is the first element's name surrounded by grave accent when printing lists?

Question

In the following list, "first" and "pio" are surrounded by double quote when the entire list is printed, apparently because they identify the first elements in their respective lists.

> list("first" = as.list(head(iris, 3)), second = list(pio = pi, poi = pi+pi))
$`first`
$`first`$`Sepal.Length`
[1] 5.1 4.9 4.7

$`first`$Sepal.Width
[1] 3.5 3.0 3.2

$`first`$Petal.Length
[1] 1.4 1.4 1.3

$`first`$Petal.Width
[1] 0.2 0.2 0.2

$`first`$Species
[1] setosa setosa setosa
Levels: setosa versicolor virginica


$second
$second$`pio`
[1] 3.141593

$second$poi
[1] 6.283185

Is the first element's name is always printed like this even though it doesn't contain a space or anything similar? If yes: why? If not what is going on? Nothing of that sort happens when printing named vectors:

> c("a s" = "hey", b = "wow")
  a s     b 
"hey" "wow" 

I'm using R version 3.5.1 (2018-07-02).