The unit with the white border is attacking the infantry unit below it. Both are standing on a mountain. If you look at the combat calculation log you see that the ranger still receives a bonus for being attacked from low ground. As both are standing on high ground, this must be a bug.
There is another minor bug around this message. If you attack a unit on low ground with a unit on high ground the message in that log will still be: Bonus when defending in high ground against an attack from low ground. I assume it is intentional that units attacking low ground from high ground still get that bonus, but the message should then be something like: Bonus when attacking from high ground to low ground.
