when you do not explicitly specify a 'order by' the result is unordered, that is, which is: data is returned as it is fetched. Normally this resultes in the order of the used index.
You may want to "explain" the statements to see, what db2 is actually performing and what indexes it is using. (db2exfmt -d database -e user -t -v % -w -1 -s % -# 0 -n % -g OTIC)