environment.xml can be downloaded at any time (from download.moodle.org that publish the HEAD version).
That introduces new checks (custom ones, relying in moodle libs) that cannot be there in older versions.
For example, the php_check_register_globals check, is under 1.9 and HEAD but not it 1.8. If, under 1.8 you try to check for 1.9 compatibility, it throws one error, because the Moodle code needed to perform that test isn't available in 1.8.
Behaviour should be to ignore checks that cannot be performed (because of missing lib of function).