Careful with the apple and oranges however
The report author is careful to make it clear that like is not always being compared with like, here. She points out that the definition of 'unplanned outage', that the IT managers questions for the report, used, were the is "Oops - bang! Is that meant to happen?" situations, as opposed to the "its Tuesday, so we reboot" cases.
Also, there's an element of perception. Different departments might be able to cope with up to 4 hours without email, whereas an IBM mainframe going off-line for a period of more than a few minutes each year, could have much more serious impact on the business. The high scores for Power series may reflect the absolute need to keep those systems up; so all credit to any sys admin, for achieving that, but comparing the management of a farm of file and print servers, with running a company mainframe, is like highlighting the difference between successfully landing a Cessna, and putting a 747 on the deck without killing everyone.
The report author acknowledges that, for some measurements, a z990, running the workload of a thousand or more equivalent wintel of x86 Linux boxes is counted as 'one unit', for reliability terms.
Also, it is a matter of perception. iSeries, and it's ilk, may rate badly, simply because - although in a lower league from the mainframes - few AS400-type boxes are running applications or databases that businesses can do without for long. They occupy that middle ground in the must-have stakes. Any perceptions of unplanned outage on iSeries, is likely to be factored much more highly than an Exchange Server being unavailable for an hour or so.
Finally, as the author also notes, the networked world we now live in, means that comparing platform with platform is not always the most robust method of determining business impact. If an AS400 is offline, a Z990 that interacts with it may be unable to do a lot of its work. The Z990 is fine, but until the AS400 is back up and running, it's just a big, black box. From a user perspective the job still wasn't getting done. It departments would do well to remember this fact.