When you colonize a precursor planet (regardless of if it has an elevator, satellites, is a prison or contaminated) you will get a special 'precursor event'. This event is <sometimes but not always> GLOBAL and is rather significant.
One of the choice is in fact a Global +15% to shield strength, others include +10% to accuracy, 10% to dmg (mass driver or beams), +10% to economy or a planetary increase of some kind. It sounds like you may have had one of of these events.
Cheers!
