Older Optimates and less sophisticated chargers charge batteries to 13.2V, but AGM batteries, like the Odyssey, need 13.5V for a full charge - the newest Optimate IIISP will charge to 13.5V, according to the Optimate website.
There's also the issue about kick-starting charge into a heavily depleted AGM battery, and I think that the 20V initial charging phase is aimed at doing that as well. The website says that it will only pump out 20V if it doesn't detect any vehicle wiring, so it looks like you need to disconnect the battery and use the croc clips to do that.
I've been using Optimates for years, partly because they are completely up-front and publish details about their charging process and have a decent set of FAQs on their website.