Gallery · FC 27 Ultimate Team
Why Optimize lowers your FC 27 Gallery score
Optimize fills each slot with the card that has the most points on its own. It doesn't count the tag bonus (First Owner, Golden, Different League and the rest), so it often swaps out cards whose bonus was worth far more. On this Starter Set, Optimize took the score from 98,298 down to 67,789.

What Optimize does
In the game, Optimize says it will replace existing players with those who have a better Gallery score. It compares card by card: every slot gets the card with the highest points.
That raises the Base Grade, here from 38,700 to 62,875. But a set's Total Score is Base Grade + Bonus Grade, and the bonus depends on the whole selection together, not on single cards.
Why the bonus collapses
Tags pay a percentage of the points of the cards that qualify, and only once enough cards qualify. The strongest is First Owner: 5, 10 and 20 first owner cards add 150 %, 300 % and 500 % of their points.
Optimize doesn't look at that. In the Starter Set above it swapped in higher rated cards, and First Owner, Golden, Different League, Different Club and Different Nation all switched off. Only Heroic was left, and the Bonus Grade fell from 59,598 to 4,914.
Before, confirm, after

One press of Optimize cost this set 30,509 points. The higher rated cards it chose are worth less here than the cheaper cards whose tags they switched off.
How to get the best score instead
Judge a swap by the Total Score, never by the single card: a swap is only better if Base + Bonus goes up.
Reach the First Owner steps of 5, 10 and 20 cards first, then look for the big point jumps.
FC-Tracker does this for you. It reads your Gallery screens, counts every tag and shows the selection with the highest Total Score, and under Best selection the exact swaps to get there.