My guess is that geologists actually find/tag a deposit halfway through the search. The two successful geologists where 1 hour long searches started at roughly the same time. The unsuccessful geologist has a 30 minute search time. I started the unsuccessful when the other two had about 34 minutes left. If I'm right, the two available mines were found 4 minutes later but the successful geologists.
Is that correct? It would explain other weird behavior I've seen when searching...
I suspect you're right. I wasn't on yesterday because I had 30 people over for my daughters 3rd birthday, so today my marble masons were screaming for marble. I sent two Jollies out (15 min search time), and the marble masons stopped screaming (exclamation points gone) before they finished searching. Today was the first time I've ever seen it.