scope of variable in prerm script

[Brian Mercer Esq <brian@mercerlegal.com>]

OK, probably a newb question:

In Ondrej's php5-fpm.prerm script he has:

[ -z "$DPKG_MAINTSCRIPT_NAME" ] && DPKG_MAINTSCRIPT_NAME=php5-fpm
dpkg-maintscript-helper mv_conffile /etc/php5/fpm/main.conf 
/etc/php5/fpm/php-fpm.conf 5.3.5-1 -- "$@"

but for mine I have to do:

export DPKG_MAINTSCRIPT_NAME=php5-fpm
dpkg-maintscript-helper mv_conffile /etc/php5/fpm/main.conf 
/etc/php5/fpm/php-fpm.conf 5.3.5-1 -- "$@"

Both using #!/bin/sh.  How come the variable is available to the child 
script in his prerm script but not mine?  Thanks.