they misheard him?

At the ninth hour, Jesus cried out in a loud voice, “Eloi, Eloi, lema sabachthani?” which means, “My God, My God, why have You forsaken Me?” '(Mark 15:35)

According to Mark, Jesus cried out Eloi Eloi, what’s interesting about this is the onlookers thought he was calling Elijah?.

When some of those standing near heard this, they said, "Listen, he's calling Elijah." (Mark 15:35)

Why are the onlookers saying that Jesus is calling Prophet Elijah  and then, of course, proceed to say they'll wait to see if Elijah helps him, if Jesus' words are asking God why he's been forsaken? 

The rest said, "Now leave him alone. Let's see if Elijah comes to save him." (Matthew 27:49)

Someone ran, filled a sponge with wine vinegar, put it on a staff, and offered it to Jesus to drink. "Now leave him alone. Let's see if Elijah comes to take him down," he said. (Mark 15:36)

If the onlookers can mishear Jesus whilst standing next to him, how could they possibly have mistaken on what Jesus had said. What’s worse is they actually  thought Jesus was calling Elijah to help him not God. could it be that Jesus did call out for Elijah but the author who wrote decades later changed it to eloi? Or else why would the Jews standing there think he called Elijah is Jesus was supposed to have preached about God Almighty?. That's the illogical part of that account. If the onlookers had understood Hebrew/Aramaic, they would not have confused God ("eli" in Hebrew or "elahi" in Aramaic) with "Eliyah" who was a well-known prophet.

what does that tell you about the authors who were not even there?. Then all his disciples deserted him and ran away. (Mark 14:50)