I've run into an issue with scatter plots in Excel. For whatever reason, Excel is distorting the appearance of markers on the plot, such that markers within the same series do not look the same. For example, in a series with circular markers, some of the markers appear as ellipses, some appear as smaller circles, etc.

Does anyone know why Excel is doing this? Better yet, any ideas how to stop this? For reference, I'm copying these graphs into Adobe Illustrator to create graphics for a print application.