Within each blog post there is an option on the right-hand side to manually set which coupon you want to show up. Otherwise by default it will show a random plugin on each blog post.
Why would you want to do this? Say you have a blog post about KeyCDN services. And you have an affiliate coupon with free credits. If you set your KeyCDN coupon to show up manually for that post, it means readers will see the KeyCDN free credits every time they read the review. This can dramatically increase your CTR and conversions. You can even take it one step further and make your coupons widget scroll with the reader.