They both started colonies at a similar time. However, I think I found the explanation, the annual wealth of the Corporate empire actually is much higher. I assumed they were the same since I hadn't intentionally done anything to boost its wealth creation but as it happens their homeworld administrator developed a massive 45% wealth creation bonus. The added production power the empire gradually developed ate up the added income, which presumably started small and grew, so I didn't notice that it had risen so much
. They were still lucky to develop the first civilian mining colony so much faster but the later ones are at least explainable by the boosted wealth.
The other empire has created its first civilian mining colony now but they are quite far behind at this point but maybe they can catch up if they get lucky with scientists.
I'll have to remember next time that a high wealth administrator indirectly boosts mining
.