The part of the slots you have in you Warehouse are the ones created automatically specifically for the items/objects you receive from the Gifts or Store windows so that you always have enough space no matter how many items you receive. However, as soon as you extract them from your Warehouse to place them on the island - the slots that appeared for them disappear too. Please do not worry, the regular slots bought for crystals remain.
Note that the additional slots are created only when you receive the items from the gifting menu or from the Store window. If you place it on your island and decide to move it back to your Warehouse later - it will require a standard slot as a regular item, despite it used to be a gift initially.
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