different BAYES_ value for same mail

[martin <ma...@excite.com>]

config is:

main mail server (said 192.168.2.5) got the email, the mail 
will local delivery and forward a copy to backup server 
(said 192.168.2.6). spamassassin log at main server shown:

======== spamassassin log at main server ======== 
clean message (3.2/6.0) for spamassassin:99 in 4.8 seconds, 66142bytes.
i: result: .  3 - BAYES_50,DNS_FROM_RFC_POST,HTML_80_90,
HTML_BACKHAIR_8,HTML_FONT_BIG,HTML_MESSAGE,HTML_OBFUSCATE_05_10,
HTML_TITLE_EMPTY, MIME_BASE64_TEXT
scantime=4.8,size=66142,mid=<0006xg2efdsyd3st>,bayes=0.5,autolearn=no

it is ok, cause the mail is a "ham" mail. But the mail at backup server 
labelled as spam, spamassassin log shown (set 6.0 as required_hits):

======== spamassassin log at backup server ======== 
identified spam (6.7/6.0) for spamassassin:99 in 3.7 seconds, 66345 bytes.
i: result: Y  6 - BAYES_99,DNS_FROM_RFC_POST,HTML_80_90,
HTML_BACKHAIR_8,HTML_FONT_BIG,HTML_MESSAGE,HTML_OBFUSCATE_05_10,
HTML_TITLE_EMPTY, MIME_BASE64_TEXT
scantime=3.7,size=66345,mid=<0006xg2efdsyd3st>,bayes=1,autolearn=no

all rules seem the same, expected BAYES_50 vs BAYES_99. What will 
the causing for this value so high at backup server. Also, i had set
internal_networks 192.168.2.0/24 at both spamassassin local.cf at 
both server, will it reduce/increase the BAYES_ value?

  Thank for helping