Champion power (and by extension team power) is affected more by certain stats then it is by others. For example, boosting crit damage and resistance dramatically increases a champions power, but boosting speed does not. This makes it quite unreliable to use when determining how well your champions will do against others.
A better approach is to look at team composition, and comparing that against yours. For example, if the enemy team has a speed lead with a skill that increases turn meter, then you will need to assume your team will be going second, so then look at the what other champions they have and the potential damage they could do and consider if this is a match that you think you can win. These type of speed nuke teams will always have a lowish power rating.
On the same note, if the enemy team has a HP aura lead, no real speed champs, and a lot of defensive champs (e.g. with skills to increase defense/heal/etc.), then it is probably a sustain team. These type of defensive teams will always have a higher power rating, but if you have a fast team that includes some sort of control, you might be in a good spot to win.
There are also teams who's composition doesn't make any sense, so it is possible that the player has just put their favourite champions in there without much thought. These teams are also usually easy to win regardless of their team power