Sometimes you get a low roll, sometimes tanks are in fortifications.

Also if the units are in a core province owned by them, they get a 15% home defense bonus.

I think Wayaya's experience above is consistent with my own. I make extensive use of tactical bombers in my game tactics, but I don't believe in using them as a one-size-fits-all hammer. I usually expect to get 1/3 to 1/2 of my tactical bombers' potential offensive hit points to manifest themselves in terms of damage inflicted on enemy ground units via direct attack, and I plan accordingly. Call it the x-factor, call it stochastically distributed results, call it the law of averages, but I have found bomber attacks generally tend to give actual damage inflicted results of around ~50% of their potential offensive strength points listed on their unit spec sheets. It's one of the reasons I have switched from mostly direct attacks when I was a rookie, to mostly "patrol" attacks as an experienced player. I have found that patrol attacks are more time efficient, suffer less defensive damage from targets' AA capability, and generally inflict more damage, more quickly when flight times are considered.

Of course, there are circumstances when lingering over an enemy ground stack is a bad idea and puts your bomber wing in greater peril, and in those cases I will use massed direct attacks of multiple 5-squadron bomber wings. Frankly I don't care if takes two or three direct attacks by 5-squadron bomber wings to a kill a medium tank brigade; by the late game I usually have 30+ tactical squadrons and I will employ chain attacks of the 5-squadron wings in quick succession to destroy high value targets when needed.