Does it really seem likely that the writers of the Bible were unable to count the number of legs on insects?

On that the Bible is not the only old book where insects legs were miscounted, as you put it. The other books have illustrated drawings to label what they called the legs instead, and the Bible does not. With out a clear cut idea as to what they were referring to, it is hard to give them the benefit of the doubt. Personally, I take it that either they were unable to count based on hunger or injury to the critter in question. Face it, if you hunt down a grasshopper, pulling a leg off here and there can and will happen. You also tend to miscount when hungry, it make it harder to pay attention.