[PvK]

If you check the replay, does the planet have a weapon platform with a Fire Control Center? These look like combat sensors, but are large and expensive and only available on large fixed defenses (planetary Weapon Platforms and Bases). They can allow heavy weapons (of types allowed to fire on fighters) to hit fighters fairly reliably. My best guess is they had one but then you took it out, spoiling the planet's anti-fighter defenses.

PvK