It is common for sales to occur, so more items will be recieved for the same prices
the Free gems from downloading Apps are not immediately available. They become available after the starter pack offer is over and after that, a few more will come every few levels.
Sometimes Com2Us holds promotions for diffrent games giving out a higher reward than what is obtainable in the Free Gem from downloading store. (The promotions during Christmas Paid 30 gems)