I saw Engine 901 the other day, which is the SDFD reserve engine covering station 35. The "901" placard on the front looked pretty temporary. It looks like they put quite a few reserve engines into service, because they needed to cover all the city engines that went out on the initial strike teams to the Bernardo, San Marcos and other north county fires.
This sounds like a normal CAD/dispatch problem that can happen when you a big fire in your area. The CAD will only let so many units work under a number. If the dispatch center is the one for the fire then they can get lost when they here engine 7, which one is it, the one on the strike team or the one covering. Add in if they were already in a reserve engine that went on the strike team that can bring up even more. I do not know how San Diego's CAD works but this seems to happen during fire season for every department sometime.