Blue, I have not noticed the US site's prices fluctuating, but the branded themes you cited already had ridiculously high prices here. Maybe branding is giving the retailer an excuse to pump up prices. I DO understand that, say, Mr Spock and Scooby Doo cost more because of licensing fees. I do NOT understand how that applies to Novelmore and Ayuma, which AFAIK are internal brands.
US site's Ayauma 70802 has been (I think consistently) $16. ...for a klicky and a bird. Compare that to 70878, for which $6 gets you a klicky and a panther. Maybe those spindly teen bodies are harder to produce.... Maybe the Ayuma paint aps are more detailed... but from my point of view, we are up-charged, quite a bit, for the Ayuma name.
Or take the BEAUTIFUL 9893 3-viking set, which is $8 here, and use Pynedor's price-per-klicky to compare to the 70672 Novelmore 3-pack of soldiers, which is $12. I do not know what the extra 50% is for, except for a blister pack and eye strain.
Have you noticed the un-branded items fluctuating on the Canadian site too? If it is just the branded items, I am sorry to hear that your Ayuma tax has gone up. :/