When Using the Mass Create tool to update your Inventory, there are three different Modes in step 3:
Create/Update Products
Use this mode only if you want to both update quantities for existing items and create new products for the entries that it doesn't find a match for in your inventory.
2. Only Update Products
This is the default mode. Use this modee to update existing products in your inventory. This is important because if there are any spelling or capitalization discrepancies between your csv and your inventory, it will skip those products and create a report of the skipped items. Choosing "Create/Update Products" when there are discrepancies will create new custom products for those discrepancies.
3. Only Create Products
Select this mode if you just want to use the mass create to upload new products to your inventory, and not update any existing items. If there are already products in your inventory with names matching the csv, they will be skipped over.