The ratio of the neuron numbers may be somewhat meaningful when comparing vertebrates with vertebrates and arthropods with arthropods, but it is almost completely meaningless when comparing vertebrates with arthropods.
The reason is that the structure of the nervous systems of arthropods is quite different from that of the vertebrates. Comparing them is like comparing analog circuits and digital circuits that implement the same function, e.g. a number multiplier. The analog circuit may have a dozen transistors and the digital circuit may have hundreds of transistors, but they do the same thing (with different performance characteristics).
The analogy with comparing analog and digital circuits is quite appropriate, because parts of the nervous systems that have the same function, e.g. controlling a leg muscle, may have hundreds or thousands of neurons in a vertebrate, which function in an all-or-nothing manner, while in an arthropod the equivalent part may have only a few neurons that function in a much more complex manner in order to achieve fine control of the leg movement.
So typically one arthropod neuron is equivalent with much more vertebrate neurons, e.g. hundreds or even thousands.
This does not mean that the nervous system of arthropods is better than that of vertebrates. They are optimized for different criteria. Neither a vertebrate can become as small as the smallest arthropods, nor an arthropod can become as big as the bigger vertebrates, the systems that integrate the organs of a body into a single living organism, i.e. the nervous system and the circulatory and respiratory systems, are optimized for a small size in arthropods and for a big size in vertebrates.