Are you playing with TZR on? If so, the arachnids should not be very common. With TZR not on, there are only two or three types of non-core fabs around in general, so they tend to be very common in those games. The "core" fabricators, and the "Experimental" fabricators are two different seed groups, and so hence why that occurs in the base game, where there are not all the new experimental ships.