Ah, the mysteries of retail pricing.
Basically you have several factors going on.
1) Supply chain. If you stock an item that is replaced on a regular basis, your price may vary rather quickly. If you sell, say, 500 cameras a month, 200 of N and 200 of C, as well as 50 P, 30 O and 20 S, but you have a minimum order for each of 200, then the prices for N and C will change every month, but the prices for P will change every 4 months, for O every six months and for S every 10 months. Hence while N and C will go through small incremental changes, O and S will jump more significantly, everything else being the same;
2) Exchange rates, one of the major reasons for the above;
3) Supply and demand. If an item is moving off the shelves as fast as it can be stocked, that is a fairly sure sign that it may be underpriced. The closer you get to the last few shopping days before Christmas, the greater the likelihood that you will find higher prices to take advantage of the fact that you have limited time to research prices. A good retailer would spike his prices on the last day of shopping before Christmas, as you can't run around looking for prices very long, and will end up paying a premium for shopping too late.
There may be other reasons, but that sort of covers the basics...
JohnF