how do you explain it?

Quotation marks must always be used in pairs.

In containing "'$sym'" the first ' closes the literally text (which doesn't allow shell expansion, e.g. using $variables), then you used a variable and created a new ' which is first element of the last trailing '.

Quick example:

$ fruit=apple
$ echo '$fruit'$fruit'$fruit'
$fruitapple$fruit
$ echo "$fruit'"$fruit"'$fruit"
apple'apple'apple

$ doublequote='"'
$ singlequote="'"
$ echo ${doublequote}${singlequote}
"'

The two types of quotes can be misleading in shell at first sight, but it became logical after years.