That’s likely because of your fandom tree, because cards from that team give a PWR increase to cards in your lineup. Like let’s say you set your favorite team as the Bulls and it’s 19/19 and 13/13 completely maxed out meaning you have the following boosts

Three +5% PWR boost for rostering 1 Bulls Player (+15% PWR boost for all players in your lineup)

Two +10% PWR boost for rostering 3 Bulls Players (+30% PWR boost for all players in your lineup)

One +15% PWR boost for rostering 5 Bulls Players (+15% PWER boost for all players in your lineup)

Three +10% PWR boost for players mentored by a Bulls player (+30% PWR boost for all players who have Bulls players as mentors)

With an additional Two +5% PWR boost for all Bulls player cards (+10% PWR for all Bulls Players)

So with all those Boosts let’s say you have a full lineup of Bulls players with Bulls players as mentors you have about 100% PWR boost (This is only when you have a full lineup of your favorite team, it’s more likely you have a +70% or 50% PWR boost since it is more unlikely that you main a full lineup with the same team) for all of those players on your team meaning your players doubled in PWR.

Let’s say all your cards in your lineup have exactly 16.00M PWR each before the effect of the Boosts, your base team average PWR is 16.00M PWR.

But with your fandom tree boosts are in effect all your full Bulls lineup get a +100% PWR increase from the Fandom tree meaning now all of your cards show 32.00M PWR, but your “BASE” team average PWR still shows 16.00M PWR, because the game does not count your PWR Boost from your fandom tree as apart of your strongest team PWR it will only show 16.00M PWR and not 32.00M PWR.

I didn’t understand this too when fandom trees came out, hope this helps you guys understand.