I do think Mike Portnoy kind of loved anniversaries, and special shows, and the science of celebration, so I think it is, in retrospect, entirely consistent with his personality that he chose the earliest possible date to count from. I would not at all be surprised if he had the 20th Anniversary setlist planned in his head for months. That said, Score at 20 does just feel right. Something about it, it did feel like the cumulative total of two decades' work, and Score feels more like the end of an era than Black Clouds did, for whatever reason. The discography felt complete, a coherent tome. "End of book one."
With it already written into the band's lore - and how! - that 2005/06 was the 20th anniversary, anything else would feel a bit funky. We've been told so publically and with such panache that 1985 is the point to start from that the shift would look weird. But when I think of Dream Theater, I do think of 1989 as the point where the band starts. It all starts with A Fortune in Lies. Everything before is history, it's gestation, the band was incubating, and gaining form. WDADU is still sort of a prototype, but it's the moment Dream Theater stopped being an idea and started being a tangible "thing," it's minute zero, so I totally get where JLB's coming from, cos I think instinctively I tend to count from 1989, too.
As I've said, though, I don't especially give a fuck where they count from! This tour, in particular, has highlighted that I really couldn't care less how old any particular album is and whether it produces an integer if you divide it by five. Band anniversaries are a bit different, because that's an achievement, that's "Holy crap, we're still doing this" - but while 2015 is thirty years since JP, JM and MP jammed at Berklee, it's equally fair to say 2019 is thirty years of Dream Theater music, and I don't really care whether they celebrate both or neither.