The default value of the variable `sentence-end-base' is:
[.?!][]\"'")}]*
There is a backslash in the second part of the regexp, why?
C-h v sentence-end-base tells you:
sentence-end-base is a variable defined in `paragraphs.el'.
Its value is "[.?!][]\"'")}]*"
IOW, the value is a string. The double-quote chars shown surrounding the string
are not characters in the string. The double-quote char shown just before the
single-quote char, ', is in the string. When printed, that char is escaped with
a backslash. The char in the string following the ' char is a different char (a
curly quote) - it is not used to indicate the string limits, so it need not be
escaped to distinguish it.