I have the exact same problem and gripe. I have posted about it in the past but all I gety is requests to post the Champions I have and the armor I have. I do not even know HOW to post all this info for close to 20 six star Champs, most with level 5 or 6 armor leveled up to 16. I can field a "Team" with 110K power and get killed off in the first round by a team that shows as 46K. Explane THAT one?? LOL one time I went in on a SINGLE 5 star L 50 legnedary with my strongest team. My whole team was wiped out with ONE shot before I do do ANYTHING. THAT is BULL SHIT.
Power is a misleading stat in arena because it doesn't consider speed, nor team synergy. In fact, if you see a team that seems to have unusually low power for the arena tier you're in, you should be very suspicious because it probably means that they are super fast, with good accuracy on their debuffer and good crit rate/damage on their nuker, so they will speed nuke you. So unless you are confident that you will be faster than them, or that you are tanky enough / have enough resistance to survive a speed nuke and then retaliate, you may want to steer clear of such teams.
I'm sure everybody posting here has experienced the same thing at some point, losing in arena to a much lower power team. But you know, we've learned from it and now know better how to assess such teams, which is why we try to offer you advice to learn from it as well - which does indeed require knowing which team you're using.
In the end, being successful on arena offense is largely a question of how well you analyze the opposing team before attacking - knowing what skills those enemies have, looking at their aura, their levels, the player's XP level, all the possible information you can get beforehand, which will tell you a lot about how the fight is likely to go. And then you have to see if that's a good fit with your own team's tactic. Of course it doesn't mean you can always predict in advance if you will win or not, but it does mean you can avoid a lot of fights that you are likely to lose - or just change the team you bring in order to reduce the opponent's strengths or exploit his weaknesses.