I wouldn't say it couldn't be the resin, but I agree with you... it's more than likely the test kits. Especially if you're reading 0.5 out of your tap. That's pretty darn high.
Just FYI... from my understanding, DI
resin that isn't sealed against air can get depleted over time from the CO2
content in the air. Depending on how the DI
cannister has been stored while waiting to be sold, you might not be starting out with a "full" charge. I've often wondered about the quality of the stuff in some of those bargains on eBay. Not sure if that's your case, but just putting it out there for something to consider.