It's tough to answer this question properly without seeing the listings in question, but here's a bit of information that may help.
The accuracy of the GSP's calculation of "import charges" depends largely on the information the seller provides on the nature of the item. If the seller provides no information on the item's country of manufacture or origin, the GSP "bot" may apply an inappropriate duty calculation.
So, without seeing the listings, I'd have to conclude that for whatever reason, the GSP "bot" figures that the laptops have been manufactured in different countries, one of them subject to duty when imported into Canada, the other one not.