Suppose, we have master branch. Then we checkout to a new branch, lets name it branch1:

git checkout -b branch1

Then we do some commits:

echo "a" > a.txt && git add a.txt && git commit -m 'Added file: a.txt'
echo "b" > b.txt && git add b.txt && git commit -m 'Added file: b.txt'

Then we push our branch to Git repository:

git push origin branch1

In Jenkins, Git polling reports:

...
Seen branch in repository origin/branch1
Seen XXX remote branches
 > /usr/bin/git log --full-history --no-abbrev --format=raw -M -m --raw 0a6eb107fea5bb6371c450db1b5f6e100e0fba28..d8c5018ca184fb75e46386de2f16e552906fe106 # timeout=10
Done. Took 3.5 sec
Changes found

but Changes section shows nothing, it means, that no changes were made. Nevertheless, build was triggered.

The problem is deeper, suppose our build runs unit tests, and they failed. In that case, no emails will be send to committers, because of no changes. We have caught this case and got a problems
I think, it's a bug, because 2 new commits were created and Jenkins should show it in Changes section.