I think that's because IBM has a massive portfolio and just treats it statistically in IP cross-licensing negotiations. I had a boss at a company who said they added up theirs and IBM added up theirs, then his company wrote a check to IBM every year.
IBM also plays tricks to artificially boosts the number of patents it gets. For example, for a computer-implemented invention, a patent application will generally have a set of claims for the method that a computer performs to implement the invention and a set of claims for the computing device itself configured to perform the invention. What IBM likes to do is to split those two sets of claim into separate patent applications, so one application just claims the method and another application claims the computing device. So when most other companies would only get a single patent for a computer-implemented invention, IBM would get two patents. So this helps IBM maximize licensing revenue in the example where they're just adding up the number of patents.
Every big corp makes such list, and measures against others, no matter what patents there are.
Nearly 20 years ago, in initial bootcamp in big-corp, there was half a day course by a patent guy, and one of the warnings he gave was "be careful where you search for your new idea" - the most wellknown patent search web site then, was owned by.. IBM.
