The strips are just generally known to be crap, really unreliable and open to interpretation as far as the colors. So not just in this instance, but for all your fishkeeping needs, getting one of the API master liquid kits is a great investment.
Your nitrate is probably fine, but it's very unlikely for it to be zero, unless the purigen really is sucking it all up. Nitrate readings of 0 are usually not considered a good thing, because nitrates are the natural result of the bacterial cycle, and if there is no nitrate being produced, that's an indication that the cycle isn't fully converting it. Again, not saying that's a problem for you, just explaining the concept a bit.