And there you have it. Instant 8% chunk, city is secure, and more than 12 hours of Eagle Storm to use.

My guess is that Joel decided to "separate" city liberation from planetary liberation, so that liberation towards a city doesn't count as liberation towards the planet, until the city is freed and then one huge chunk of liberation is added to the planet. Everyone on Alta was in the city, which explains why the planetary liberation rate was so low, but now that Porte Liberte is free we're back to non-megacity gameplay, which means liberation rate is going to shoot back up after the recalibration.