write-to-string

[kevin@freya.cs.umass.edu]

WRITE-TO-STRING takes a number of keyword arguments (e.g., :escape)
that control the printed representation of the objects output to the
string.  These all default to the corresponding global control
variables (e.g., *print-escape*).  Because you didn't specify the
:escape keyword, in your call to write-to-string, it's output will
change depending on the value of *print-escape*.

You should do the following instead:

  (with-standard-io-syntax
    (write-to-string ...))

This  will ensure that strings are printed with double quotes, numbers
are printed in base 10, etc.