That five-fight win streak of Arlovski's came against Roy Nelson, Ben Rothwell, Jake O'Brien, Fabricio Werdum, and Marcio Cruz. The only notable name in that crowd is Werdum, and Arlovski's win over him was a lackluster decision.
And yeah, Sylvia's losses to Big Nog and Couture aren't terrible (although Sylvia was never once in that 25-minute fight with Couture), but look at his recent wins: lackluster decisions against Arlovski, Jeff Monson, and Brandon Vera.
Over the last three years, three bad wins over good, but not great, competition, and two losses (including one terrible one) to great competition, doesn't make for much more than a gatekeeper at this point.