Just off my own experience living for a year in Vietnam, which was far less sophisticated at this than China... once your apartment has raised red flags, any suspicious traffic is going to lead to a temporary 15-minute to 1 hour internet shutoff. Any ports or domains you accessed will be blacklisted to trigger longer shutoffs in the future, just in case you didn't get the message the first time. The third or fourth time, someone from the police will politely ask you some questions. (I work for a western corporation that requires a VPN and is considering investment is the correct answer).
Maybe we're at a point now where traffic analysis weighs into the decision-making process, but that would actually mean freedom of information was winning and breaking through if they had to winnow down their target list that way.
In other words, there's no rhyme or reason to one service or IP being blocked and another being left open. It's done completely by fiat with the main intention not being to prevent you from access, but rather to frighten you so you censor yourself and think thrice before trying to connect again.
