RAM slots in almost all computers has a certain arrangement in which they need to be populated. In standard ATX-like motherboards with 4 slots you put a single stick in the 2nd from the left slot, two sticks go in the 2nd and 4th slot and four sticks fully populate the slots. In the mac mini, they way you describe it, a single DIMM should populate the top slot and two DIMMs populate both slots. It’s just a wiring thing in the motherboard.