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So I was scouring the older posts of the Onell blog for fun and happened to come across the answer to a question I'd had for quite some time - why do some travelers wear different armors. Turns out the original Exellis chest was prototype Flexion armor similar to those that glyans use in the present. Since travelers are upgrades of sincroids it would make sense that a few of them chose to continue wearing it. Eventually the sincroids would be given updated armor, too, which was the Dark Traveler armor; an obvious upgrade considering Glyaxia's tinkering with their programming to make the Dark Travelers.
tl;dr: its okay to dress up your robot mans the way you want to.