The default setting for the Upsell recommendation box is showing the Most Popular Items on your store as a fallback since you have not picked your Upsell Items for your products.
You can change the setting to show 'Frequently Bought Together' items, 'Trending' items or 'Related' items. Also, changing the Item Ordering to Price High to Low.
These can be changed in your LimeSpot Personalizer admin panel's Box Designer.
Tip: Picking the upsell items for your products will be more effective since you know your products best. Go to this article to learn more about this.