The way Apparel works is that each time you receive an apparel item in your sponsorship box it also has a numerical POINT value (1-50) depending on the level of the box. As you accumulate those Points you will see the split number increase.
Each apparel item has 10 levels which you can unlock by first getting enough points for the next level and then, through your Edit Apparel in your profile upgrading that item. This does cost Silver coins as well but will increase the abilities of the item and depending on the level unlock different colors.
The split number of 160/2 for example means that to upgrade to the next level you need 2 points and you have 160. If you upgrade the item it will change to 158/10 (10 points required and you have 158) This continues all the way to level 10 which requires 1000 points. If you see an item with a split like 25/100 then that means that you need 100 points to upgrade but you only have 25 and will need to accumulate 75 more points.
As far as how apparel is awarded, you chose which sponsor you wish to receive apparel from. There are 5 sponsors and they need to be unlocked by playing games with the sponsor active. You are only guaranteed 1 item from your chosen sponsor and any other items can be from any of the 5 sponsors. Which items you receive is totally random.