I'd just like to highlight a mistake I made in the compression test. I was re-reading through the manual again and happened to come to the section on the compression test. I noticed that I missed out something.
The manual says to hold the throttle wide open while cranking the engine over. I forgot to do that.
Being curious, I wondered if it made a difference, so I removed one plug and hooked up the compression gauge and gave it a try. I got a reading of 150-160PSI (contrast that with past readings of 90 - 100PSI). So apparently it does make a big difference.