The reason is Maven sorts strings literally, thus java10 would be collated before jre9
I don't think it will matter as the release version (ex: 9.4-1234) is prefixed to the full version anyway Newer versions of any JRE would appear "newer" than older versions of a later JRE (i.e. 9.4-1400-jre6 > 9.4-1300-jre8).
Using two digits won't fix anything and just looks odd compared to how JRE versions are generally referred.