Rough in size is often misunderstood. We are accustomed to quoted size numbers to be a finite number, but this is not true with toilet rough-in. It is often impossible for a plumber to center a toilet flange at exactly 12" from the finished wall as the frequently is a floor joist that prevents it. So, the flange may be plus or minus a little. Fortunately, toilet manufacturers allow for this when they cast the their toilets. A 12" toilet does not require the full 12". There is always space between the tank and wall. In your situation, this means a somewhat large space, but sometimes the flange may be less than 12" and in that case, the 12" toilet may still fit, it will just be closer to the wall. Sometimes the offending floor joist is is such a bad position that the flange can not be even close to 12", and that presents other problems that have to be dealt with.