Could it be that these Sunnyvale developments were just issued permits earlier (2016 or 2017), and we're just seeing it further down the pipeline?
Sunnyvale has been reliably pro-housing since about 2010, which means that a lot of new supply came on the market in 2015 and 2016, which has alleviated the housing supply issue significantly. I left MTV in 2014 after having my rent double in 3 years, from $1400 to $2800; my rent in Sunnyvale has gone up by about $30/month total in the 5 years since, and the 2 BR townhome I'm in now currently rents for less than the 1BR in MTV that I used to live in.
Put another way, restricting housing supply in Mountain View likely impacts rents its socioeconomically similar neighbor, Sunnyvale.
Among likely differentiators: 1.) Traffic. If you're traveling to Google or Facebook Sunnyvale adds an extra 15-20 minutes to your commute. 2.) Schools. They're better in Mountain View, at least for now, though both regions are rapidly gentrifying. 3.) Cachet. Mountain View has the name because of Google. Sunnyvale doesn't really, and also feels like more of a sterile suburb than a lively community. 4.) Shuttle stops. I don't know exactly where they are now, but that'd explain why the downtown areas go for relatively equal expensive rents, while my neighborhood (which AFAICT has no tech shuttle stops anywhere near it) rents for a lot less than other neighborhoods in the area.
I definitely don't have a nuanced enough understanding of the difference in civic liveliness of the two cities - and resulting desirability - they both always felt pretty suburban to me. But I've never lived in either one.