There are a few different things that can cause this:
1. Uneven rib cage. It's possible that the right side of your rib cage could poke out slightly more than the other side, pushing those muscles out further than the other side. I have this issue with my ribs under my upper abdominal muscles, causing a slight bulge (virtually unnoticeable to anyone but me).
2. You're right-handed. If you are more coordinated with the right side of your body, the muscles on that side of your body will generally be slightly larger. For me, no matter how big my muscles have been over the last 12 years of weightlifting, my right arm, right thigh, and right calf muscle have been a half inch larger than the same muscles on the left side. I imagine it's the same for my chest but I don't have a good way to measure the difference.
- You could be pushing unevenly. If you are right-dominant, it's very easy to push more of your body weight with your right-sided muscles because they are stronger, and the movement is easier to perform that way. If you go to a gym, you can often see people bench pressing unevenly in such a way that more of the weight is applied to the stronger arm.
The cure: Be conscience of your body and how you're pushing your body weight. Try to engage both sides evenly. If that still isn't enough, shift your body weight closer to your left hand to increase the load on your left pectoral muscle. When the left pectoral has reached complete failure, stop, even if you could still push out another rep or two if you shifted your weight toward your right hand.
Also, don't just do one pushup variation. Do your research or do some experimenting and find 10-20 pushup variations if you want to build a full, strong and even chest.