I think the figure of 957 may be the total number of aircraft that have ever held an IOM registration, many aircraft are registered for just a few days until the deal goes through, then go onto another register.
I think the actual number at any one time is around the 500 figure.
There are many reasons for registering aircraft on the IOM register other than tax avoidance, a high number of business aircraft based in Israel are IOM registered for ease of overflights in the ME, ditto aircraft in Cyprus flying to Russia, if registered with Cypriot registration they could not overfly Turkey.
One factor of operating a private aircraft on the IOM register is the pretty good service provided by the Registry, they actually answer the phone and help sort out problems.