Here are some pictures.
https://www.bing.com/images/search?q=stomach+diagram+anatomy&qpvt=stomach+diagram+anatomy&FORM=IGRE
I think its an issue of where the body of the stomach sits when you lie down. If on your right side, gravity drains it into your intestine. If on your left, your stomach has to push stuff uphill into the small intestine and its more likely to backup into the esophagus which is on the downhill side.
Thanx pete
food stays in stomach for 60-120 minutes before slowly released in intestine, unless the sphincter at stomach exit is damaged. All that time there is full amount of acid in stomach. Most people with acid reflux have a poorly functioning valve at stomach entrance. That is why sleeping on left side helps. Because stomach entry is on right side.
At the end of day it all depends which part of stomach has issue, entry or exit. Eack person will need to find out for themseves.