The data is not the same between the platform and the platform. This feature is mainly to serve the limit of Shopify variants.
On Shopify, We can create up to 100 variants for a product, but AliExpress can create products with more than 100 SKU, or called variants on Shopify. So if you want to sell all SKU of the product on your store, please split the product.
Before importing the product, in products' more action. All the products in not imported list can be splitted.
Split by options will split the product according to options. For example this product can be split to 18 products by "Color" options.
Uncheck some color variants will keep it in the original product.
Check the "Remove the original product after split" will remove the original product on list.
This option will split the variant check to another product. So if you haven't checked "Remove the original product after split", the product will split in 2.