When picking a new reinforcement ship for the central pulse or a non-special-forces guard-post-pulse, it has a 10% chance to pick any available ship like usual, and a 90% chance to pick one of its focus types. When picking a focus type, it has 3 times as much chance to pick the primary as the secondary, and 3 times as much chance to pick the secondary as the tertiary. If that doesn't make sense, don't worry about it.
If I'm understanding correctly, that's a 62% chance to pick the primary, 21% chance to pick the secondary, 7% chance to pick the tertiary, and 10% chance to pick whatever. For an AI with the three triangle ships and two bonus fleet ships, the 10% presumably consists of a 2% chance of picking any of the five available fleet ships. So the overall chances of picking the different ships would work out to 64% / 23% / 9% / 2% / 2%.
It's not purely random, sometimes it picks ships that fit certain roles.
Mystifying! Looking forward to discovering what this means