Google cannot know if you already bought the RAM, or were still merely considering buying it. So it looks like they assume the latter, just in case.
I was shopping for a simple necklace for the femm-bot and found an acceptable one at overstock. For at least the following month, I was positively bombarded with advertisements for that necklace. It was particularly annoying, since I had already bought it.
(I don't actually know which ad network I was being harassed by, it just happens to be the only long-term advertising I recall being exposed to)