Comment 5 for bug 1525395

The responsible code is here: http://bazaar.launchpad.net/~widelands-dev/widelands/trunk/view/head:/src/logic/production_program.cc#L1239

A penalty for empty or nearly-empty fields is added, so I guess the chance of extracting the very last bit of resources is very low. I do not know if this has to do something with the peripheric position (I don't think so), it might have been luck that the second mine digged out the resource immediately.