There's a bit of a story there. When we started preparing the 5.10 CDH release, the Apache 2.8 Impala release was not ready, so we had to call it "Impala 2.7" in the version number. Impala 2.8 was officially released after we finished putting together the CDH5.10 release - too late to bump the version in all places.

CDH5.10 Impala is almost exactly the same as 2.8, plus or minus a few patches, so in most of the announcements we've just called it 2.8.