Muscle fibers are made of long strands all running in the same direction, we call this the Grain. Jerky is sliced with the grain or across the grain. When you slice an eye round across the grain you get round slices with the muscle fibers cut so you have millions of 1/8" thick threads bound by connective tissue. Chewing breaks these up easily and we say the meat is tender. In contrast slicing along the length of the eye round, you get 4" X 12" long sheets of long muscle fibers all running the same direction. Bite into these and you are not breaking up tiny fibers, you cutting through the long fibers like trying to bite trough a bungy cord. Tough stuff!
In general, Commercial jerky processors cut with the grain, long sheets, because for a given weight, there are fewer pieces to handle. The Dry sheets are cut into the long extra chewy strips you see in jars at the counter of the gas station. If the processor cut across the grain there would be billions of little rounds to process. Too much handling and not cost effective.
As weekend warriors, we are only making jerky from a few pounds of meat. We can cut across the eye round, little circles of meat, because we are not making a lot. This jerky cut across the grain is more tender the the long strips....BUT....If marinated too long, the connective tissue is dissolved, or dry too much, becomes crumbly, you get a mouthful of Sawdust not a slightly chewy but still tender jerky. Meat cut With the grain, long strips, holds up better to long marinating or thorough drying. The long fibers don't fall apart as easily...JJ