The inquisitors had a lot of spikes so it could probably be possible that they just had a different spike in their back. The Hemalurgic Table had a map of Inquisitor spikes in the body but it didn't show which were in the front vs the back. If I were to wildly speculate though, the Hemalurgic Linchpin for the Inquisitors was a steel spike (A spike made of steel, but also the spike that steals physical allomantic powers) located in the upper back / lower neck, so could it be possible that that was where Zane had his spike? In which case, it would be much more important to have the linchpin spike there than it would be to have an A-Steel Spike.
On second look there actually is another spot in the back for physical spikes, which means that the argument is kind of moot. However, looking at the diagram of Inquisitor bind points (Which isn't every bind point but still probably useful) there seems to only be a couple of spots for physical Hemalurgic spikes outside of the eyes, so its possible they may have already been using those for other attributes? (A-Pewter, A-Tin, F-Pewter?, F-Steel?)
