Example:
This link, to the black version of the product:
https://www.ebay.com/itm/181675017297?var=480631203366
Will work on Desktop and on Mobile browsers, showing you the black item.
but on eBay app it will take you to the default item (as if the link was just https://www.ebay.com/itm/181675017297)
so user have to manually find the item himself.
Conversion ration dropping.
Now Imagine this when there are even more options user have to choose from to finally find his product.
