If the blue energy bar works the same way it did in GLB1, then the bright blue with the number that drops during your play is your energy for that moment. The dark blue in the background is your maximum energy at that point in the game. You start every game at 100 max energy, and as you play you lose energy, which recovers considerably between plays. However, max energy depletes steadily throughout a game depending on how often and how much you're burning your energy down during plays.
Example (numbers made up):
Start of game 100/100 energy. Then you take part in a play that drains you to 60 energy, but also perhaps drops your maximum energy to 99. A sub comes in for you, so you sit and recover your energy to 99/99. The next play you're in you drain to 80, and maybe that doesn't impact your max energy, but you don't get a sub, so you only recover some of it between plays, maybe to 92/99. Then you do the next play and it drains a lot of energy down to 30, and also damages your max energy to 97, leaving you at 30/97, and a sub comes in to give you a play off. This allows you to recover to maybe something like 82/97. So energy fluctuates between and during plays heavily, but your maximum energy steadily depletes throughout a game.
If your maximum energy (dark blue) bar is different from your teammates, it's due to many things. # of plays in which you're player was involved. Conditioning/Toughness skills. Opposing player intimidation. What type of physical activity you did during a play (blocking, tackling, etc). The formula for how much light blue energy you have to lose to inflict damage to your dark blue maximum energy isn't shared by the developers so far as I'm aware.