The logic is half bits, and half timing. Similar processing power is grouped together, so that you have 16 bit consoles, starting with PC Engine, starting a new era. Likewise, graphically, 7800, NES, and Master System blow out anything that came prior. But, they don't really go with 1986. They go with 1983 when the Famicom was released. 1986 Famicom, 1984 7800, 1986 Master System, and the generation ended in 1987 with the release of the PCE.
As far as the ColecoVision goes, when you really think about it, until the PCE was released, all consoles were just slightly better than their predecessors. Generations is a hard thing to tackle. That's when timing plays a part. The CV died just a year after the Famicom released, and wasn't really competing with it, but rather with the 5200 and Intellivision. So people naturally group those three consoles together (and oddly and illogically enough with the 2600, too) and exclude the Famicom, whose lifespan was from 1983-1991 (and even later, if you want to be technical). Also, it didn't really have much competition in America until mid-way through 1989, and then even then the Genesis only gave mild competition through lack of excellent games, at which the NES really succeeded.
Not saying it's right or wrong, just giving some common reasons.