Bearing terminology can be confusing. Terms like "undersized" and "oversized" often mistakenly get used interchangeably. It's easier to understand if you speak of bearing sizes being "thicker than stock" or "thinner than stock"
Sometimes a crankshaft will have imperfections on the journals that require machining to correct. A machine shop will remove the layer of imperfect material from the journals, leaving a fresh surface that can ride along the bearings better.
When the material is removed from the journal, the journal then becomes smaller. These are the cases when a "thicker than stock" bearing is used to make up the difference of the removed material. For example, if 0.020" of material is removed from the crank journal to get it right again, then we need a 0.020" thicker than stock bearing to fill the gap.
Another case for a non-standard-thickness bearing is to adjust oil clearance. These bearings are only thicker or thinner than stock by a very small amount (0.001") and often don't require machining of the crank or the block or the rods.
A bearing that is 0.001" THINNER than a stock bearing is called an "Extra Clearance" bearing because it allows for 0.001" more oil clearance.
A bearing that is 0.001" THICKER than a stock bearing would REDUCE the oil clearance, making it tighter.