Because the counter stops at 99!
This number only reflects the amount you have in Harvest & Products storage. Once you reach over one hundred crops and/or products in stock, the number is replaced by ++
Note: The number of items stored in the Items Storage doesn't influence this counter. For example, If you have a hundred unused Items in your storage such as buildings/decorations but you only have 3 boxes of apples in storage, the storage will show "3", not 103.
To find out more details about storage read
Chapter 14 - Storage