The topic pretty much explains it all. It’s happened that way for me once, I wanted to make sure I knew how it worked though. Does the higher level ark determine it? Is it random? Something else?
Based on my experience…yes, it will always form a S.
Yes it will. There it an easy way to tell what grade ark you’ll get, it’s just like maths:
if E=1, D=2, C=3, B=4, A=5, S=6 points;
Example: You combine A and S, you will get S. Why? 5+6=11. 11/2=5.5 As long as you get .5 or higher it will automatically round to 6. 6=S.
Another example: You’re doing OM and you need 5 arks. You manage to get 5 from eggs that are C, A, S, S, S. What do we get? 3+5+6+6+6=26. 26/5=5.2. 5.2=A
Pretty easy, just need a calculator for some of them 
HunterEdik is correct. i have tested this multiple times with nearly every fusion, save for a few OM. I don’t know why the 50/50 thing has spread so much. I have fused S and A probably 40 times by now, and it has ALWAYS made S.
…you know, I’m just gonna make a thread for this.