The invasion's ground strength (in terms of raw marine numbers) scales partially to the target's expected garrison size (before the fleet does its tactical bombardment on arrival thing), so the estimate should start out about even. Though it doesn't take into account the admin's Ground Operations bonus (if any), and while it considers the drop attrition effect of ground defenses/heavy batteries I don't expect it's very good at it.
Next time I'm working on the mod, I should probably look at a breakdown of the estimate calculation and see how it can be improved.
Hmm... so I played around with it a bit more.
Requesting 100 FP, 100 marine invasion on Lucifron (1804 space, 39312 ground)
Results: space forces outmatched, ground forces outmatched
Requesting 1500 FP, 16000 marine invasion on Garnir (16 space, 77 ground)
Results: space forces superior, ground forces "evenly matched", outcome is uncertain
It seems like no matter how much ground superiority you have over the target, the scaling that you mentioned makes it so that your ground forces can never be listed as superior to the target's. This means that the tooltip will never say that an invasion is likely to succeed, which can be frustrating when an enemy faction is invading your colony and you don't know if "evenly matched" really means evenly matched, or if they actually have 100x your ground forces.