Why is it that when I get a free box of spare parts in Train Station 2 and I land on parts for Common..

Why is it that when I get a free box of spare parts in Train Station 2 and I land on parts for Common Trains rather than the quantity adding to the quantity I already have most of the time the number I end up with is LESS than the number I had. For example I had stock of 1280 common train parts, I landed on 110 common train parts which should have taken it up to 1390. However the quantity I ended up with was 569! How is that possible? Am I the only player in the world that this happens to?