From some recent experience I do think timing has some influence, when I went to A&E by ambulance, not my choice btw, early last year, they turned up within minutes in our rural area, however the Sheffield A&E was madness, we had to stay on the crews trolley next to the doors, we didn't even get inside for about an hour and a half, then we were waiting in a corridor for another couple of hours. Anyway the point being all this time it was on the ambulance trolley, not only that but they had to stay with me, I wanted them to go but they couldn't, and what with lots of crews there your thought was how many crews are left to respond to 999 calls. They said themselves that this waiting in A&E had a serious impact on responding.
I later learned that that night was one of the worst A&E had seen in a long time, it was crazy, a different day could see it not so busy so more crews to respond. Some of it is pot luck, plus you only read about the problems, I can't say without the stats but maybe most of the time responses are within time, though I'm sure city's will be at risk of longer respond times than rural.