Shimano Biopace.
This explains the differences;
"The oval chainring of the late 80's was from Shimano and termed, "Biopace." These fell out of favor due to having the large diameter in the wrong position.
This Wikipedia article essentially presents the crux of the Biopace story in the first three paragraphs.
There is now a resurgence of oval chainrings designed differently than Biopace. They are marketed toward mountain and gravel bike riders. Absolute Black and Wolftooth are two leading manufacturers of oval chainrings and both claim abundant research behind their offerings. Their oval (aka: elliptical) chainrings "have the smaller effective chainring diameter coincide with the cranks being at the top dead center (TDC) and bottom dead center (BDC), thus making the crank easier for the rider to turn through bottom dead center for constant chain tension. By having the chainring at its peak effective diameter with the cranks level, where the rider has maximum leverage over the crank during the power stroke, these designs are supposed to make better use of the rider's power output." (Quoted from Wikipedia).