$spam_score value change in exim + spamassassin

[Mark Copper <mlcopper@gmail.com>]

Would anyone know offhand what might be changing the value of variable
$spam_score in the acl/40_exim4-config_check_data section of the exim4
configuration file exim4.conf.template?

It's an idle question but I'm curious why I would be getting a header like this:

X-Spam_score: 10.8
X-Spam_score_int: 108
X-Spam_bar: ++++++++++
<snip>
Subject: ***SPAM (score:8.3)*** Is Donald Trump Headed For A Heart Attack?

when the text of the config file looks like this:

warn
   spam = Debian-exim:true
   add_header = X-Spam_score: $spam_score\n\
             X-Spam_score_int: $spam_score_int\n\
             X-Spam_bar: $spam_bar\n\
             X-Spam_report: $spam_report
# add second subject line with *SPAM* marker when message
# is over threshold
  warn  spam = nobody
      add_header = Subject: ***SPAM (score:$spam_score)*** $h_Subject:

So something changes the value of $spam_score between the two
warnings. How does that happen?

Thanks for looking.

Mark