What a delightful design! I agree that the “other designs” that are being presented aren’t very relevant, but my guess is that it’s because there are similar words in your tags and their tags.
Your design uses the following tags: sheep, cute sheep, ewes, fluffy sheep, festive, sheep art, sheep puns, farm animals, christmas sheep, watercolor
The first “other design” uses the following tags: christmas, nativity, baby jesus, jesus in manger, farm animals, donkey, sheep, chickens, holy night, silent night
So the following words show up in the tags for both products: christmas, sheep, farm animals
BTW, I don’t think it’s a good idea to repeat the word “sheep” so many times in your tags. I’m not sure if that’s considered tag stuffing or not, but at the very least it’s a waste of tag space. If I was tagging that product I’d probably try to fit a bigger variety of words in so you’d have a better chance of showing up in more searches. Maybe something like this? “we wish you merry Christmas” “funny ewe pun or joke” “holiday humor” “cute fun design” “watercolor sheep or lamb” “colorful strings of lights” “personalized with name” “whimsical and silly” “festive quirky and unique” “adorable farm animals”
I can’t claim to have cracked the Zazzle search algorithm, but I’m pretty sure the customer doesn’t need to search for an exact match of your tag phrase in order for it count as a match. So while an exact match probably scores higher, if they type “christmas sheep” it would still show up somewhere in the search as long as both of those words are present somewhere in your tags - even if that’s not one of your tag phrases. I think it’s always a good idea to fit as many different relevant words as you can into the tags to give yourself the best possible shot. I could be wrong, but that’s my strategy! Hope you sell some - the design deserves to be seen!