Sharing 28.3% with first cousin. What does this mean?

So for the Father’s Day special, I decided to get myself and my cousin a kit.

We were both really excited to get our results in because we wanted to see if our paternal line (our dads are twins) was completely Russian, since our great grandma was adopted and had been told she was Russian and married a full blooded Russian. Our grandma then married grandpa, who was also fully Russian. Our grandma passed away before we could test her unfortunately so we had to settle with testing ourselves.

My ancestry composition showed 49.8% Eastern European and hers showed 49.4% Eastern European. Both of us had 5 dots for Russia and 1 dot for Ukraine.

We went to look at relatives to see if we could find close matches that could help us find our great grandma’s biological family.

We were expecting to see each other at the top, but not sharing as much DNA that we do. We share 28.3% of DNA. It says “We predict (cousin’s name) is your half sister.” I guess because we are both 24 and are close in age. I looked up a chart for help but obviously it didn’t do the trick and I need more help. From what I understand, aunt/uncle and niece/nephew, grandparent/grandchild, and half siblings all share this amount, but we aren’t half siblings. I’m confused. How can we share such a high amount to be only first cousins?