Indeed a year divided by 4 is 13 weeks per quarter. However Tesla has shutdowns or slowdowns all the time, and most manufacturers are down for one reason or another, so it's more reasonable to use 12.
In Tesla's case, they commonly shut down in the first month of a new quarter. They crank production to a high rate to blast cars out at the end of the quarter, then announce the high rate they're almost at, we all get excited, and then in the new quarter they take a breather to assess the new production process, fine tune things and then resume. This was very pronounced with the X launch, but early quarter shutdowns are the norm to some extent. It was probably only 2-3 weeks ago that Tesla really got blasting on Q3 cars at a rate >2000.
Unlikely that Tesla had any shutdown in Q3 due to need to ramp Model X production to 1,000/week. Below are quotes from the Q2 letter and conference call transcript (published August 3rd) that reflect on the production rate one month into Q3 and projections for the second half of 2016.
Letter:
"Production and demand are on track to support deliveries of approximately 50,000 new Model S and Model X vehicles during the second
half of 2016."
Conference Call Transcript
Musk:
"So, I mean, I think in sort of a nutshell, one way to think about Tesla right now is that we're right around 2,000 cars a week and we're trying to balance the mix to be roughly half Model X and Model S."
"But a high-level overview is just we see demand being fairly strong at an average of 2,000 cars a week and we're able to maintain production at that level, notwithstanding occasional supplier hiccups. And then hopefully, we can grow that a little bit towards the end of the quarter and then a little more in Q4. And our aspiration that's unvarnished here, this is just what we're aiming to do internally is to do a little better than 2,000 a week in sales and deliveries in Q4, combined Model S and Model X."
Wheeler:
"But now we're pretty stable at the 2,000 cars a week level. And every time we (22:42), it's getting better. So that each passing week gets better and better."
Musk:
"And then we just managed to climb out of hell in like basically partway through June. And now the production line is humming and our suppliers mostly have their *sugar* together. There's a few that don't. One I'm going to be visiting on Saturday personally to figure out what the hell's going on there. But we'll solve it."
"So, yes, with that said, production is like, it feels like we're – I'm not losing sleep at night literally because of production issues right now. 2,000 feels like a good number with a slow, steady increase in that number,..."
Wheeler:
"Yes, I think, Elon, I think you covered it well. In terms of the modeling question, yes, I think just extrapolating from where we're at now, we're stable. We'll continue to get better on production throughout the course of the year. We've got a couple more holiday weeks in Q4. You might want to think about that when you're doing your modeling. But, yes, I think Elon covered it well."