Those are basically the stats +- of when I found each summer abra. You can see that some times you find the form easier than other times. It's all just based on luck. I had 4-5 boxes of scyther before I found a summer one too, so if you count how many pokemon can be stored in one box multiplied by 4 or 5, that should give you the encounter estimate of scyther more or less.
Regardless, everything is luck based. Yes it will be frustrating and demotivating but you also have to take breaks in between. Believe it or not, I left the game and watched anime here and there and went back to hunting. So yeah, I hope and wish you luck o/ A never give up spirit helped me 😮