I've paid attention to when this happens, and the only situations I've encountered in which these streaks award no XP is in bossfights or events, where the minions you slay are summoned as a part of the event/bossfight, and award no XP at all.
Since you mention that this has in fact happened to you outside of any kind of event or bossfight, I think it's reasonable to assume that this applies to monsters which award no XP in general.
This shouldn't apply to monsters which are simply much weaker than you, since you can, for example, go back to Act 1 with a level 30 character and still get Massacre and Mighty Blows which award XP just fine.