I think I found the issue with the numbers sometimes being different. The data comes back with dew points, temps etc for each 5-minute interval with a low, high, and average over that period of time. I was taking the highs and their website was taking the averages, so I've changed to also look at the averages (apart from for places where it's specifically looking for highs and lows which it then uses those respectively)
I'll get an update out soon.