Because that's how it is unfortunately. There was a time when android players didn't get goals at all. The reason given was some high tech explanation but what i gathered from it was that there were too many different types of android devices and it was hard to design the goals in a way to make them available to all the devices. Or some such thing. There's a thread here somewhere that goes into it in detail.
Anyway, ios players used to get different versions of the game with updates and bug fixes and whenever we got updated versions to download we would receive a goal which started out by prompting us to update and when we completed that goal we would receive some gems usually. This doesn't really happen anymore...or at least the updates are less frequent now.
What the tech people/developers did to get around the android problem was to create different versions of the game for android players and, I suppose that as compensation for the hassle of having to download new versions each time, mini goals will appear in the game prompting users to download the newer version of the game and upon completing this mini goal, gems would be the prize.
Android players usually get updates by Friday and if you wait until the following Monday (I believe) you will usually get the goal prompt which starts out as a challenge for you to download the newest version of the game and upon completion of that goal then you get gems of varying amounts, but usually six.