I got an 15.6" HP Notebook a few years ago and it had 3 inexpensive upgrades offered, HDD to 650GB from 500GB, Bluetooth Wireless and 6GB RAM versus 4GB. It was an interesting purchase, bought online from HP, got an E-Mail 2 days later that it was shipped, got it 5 days after that. Most interesting part of all that is that it came directly from the assembly plant in China, never actually was in a US facility of HPs.
It's a way of getting rid of excess stock 2GB SODIMM memory modules - which are otherwise pretty near useless when more than 4 GB is standard - to offer an inexpensive U/G to 6 GB from 4GB.
FYI... Some laptops actually do have RAM modules soldered in place. My Asus Zenbook Prime laptop DOES have it's modules soldered and in an thus can't be upgraded, therefore I'm stuck with only 4-gig of RAM.
Asus Zenbook Prime - UX31A (Service) - "RAM upgrades are not possible as they are soldered onto the mainboard to keep things compact"
But yes, some laptops actually do have RAM soldered in.
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What is strange, though is that it came with only 6 GB. Why on earth did they put in a 2 GB and a 4 GB! Why not two 4s, since it's pretty much the same price now days.