multiprocess and special bindings

CL-USER> (defvar *a* 1) 
*A*
CL-USER> (let ((*a* 2)) (sb-thread:make-thread (lambda () (print *a*))))

1 
#<SB-THREAD:THREAD FINISHED values: 1 {100482B0C1}>
CL-USER> (let ((*a* 2)) (when (zerop (sb-posix:fork)) (with-open-file (f "/tmp/a-value" :direction :output) (print *a* f))))
NIL

$ cat /tmp/a-value 

2