[racket] constant vs. variable behavior

In the example as given below 'a' behaves like a constant whereas the value named with 'x' can be modified (in the interaction window) as expected. Could you help me please to understand what happened by (set! x "Bob")?

Definition window

#lang racket

(define a "Hello")

(define x "Hello")
(set! x "Bob")

Interaction window

 > (set! a 3)
. . set!: assignment disallowed;
  cannot modify a constant
   constant: a
 > (set! x 9)
 > x
9

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