This was actually answered by the first episode of Enterprise, when Malcolm Reed came in with a box of new phasers he'd been keeping under his bed or whatever. It turns out that Starfleet ships keep their newest equipment locked away until it's dramatically appropriate.I'm particularly surprised because starting with VOY season 2, they used the updated phasers (curved grip ones) and tricorders (flatter ones) that DS9 started using in their 4th season.
Obviously, they had no contact at all with Starfleet until much later (season 4's "MESSAGE IN A BOTTLE"). So how did they get the updated equipment?
I've been very negative about DS9 in my comments so far and I'm afraid I'm going to have to do it again, as Rivals is one of the few times where the series failed to sell me on its reality. There's a lot I can go along with, telepathy, godlike aliens, transporters etc., but the idea that a probability device can affect luck is pure fantasy, as in there has to be intelligence behind the magic for it to work. Something has to understand what tennis is before it can precisely sabotage one player in multiple different ways. So I didn't buy it. Also it wasn't a very good episode in general.
(I'd also give it a 6 though).