It is probably as simple as Google beat expectations while Apple missed expectations, these expectations being priced into the share price before the announcement. The share prices are now just correcting based on the actual results.
Over the past year, Apple has shown (slight) vulnerability, whereas Google has looked like an even more formidable competitor moving forward
What is this assumption based on?
Right. Isn't stock also an indication of future performance expectation (at least in some part)?
It's ENTIRELY an indication of future performance expectation.