TL;DR: Combination of silly operating rule, unskilled operators, and incompetent supervisors.

Prior to 2009, the trains operated in automatic mode. When in auto, six-car trains would stop at the center of the platform rather than the departure end. When Metro switched back to manual, some operators would somehow "forget" they were driving an eight-car consist and stop the train at the six-car marker -- leaving some of the cars still in the tunnel when the doors opened.

The response from Metro's safety department was to stop all trains at the eight-car marker regardless of length (or how inconvenient it is for riders, particularly those platforms like at Chinatown).

The annoying part about the policy is that the operator's console actually has a very prominent display that indicates the train length. That pretty much makes it impossible to "forget" how long the train is. Even worse, the trains have a system in place to detect when all cars are within the platform limit. It's part of the design for the automatic door system. But even when both train and door operation are fully manual, the operator can push a button on the console to force the check. It literally takes one second to push the button and wait to see if confirmation light (Back End Check) comes on.

The operators make those tiny adjustments because they're afraid of being written up for violating the policy. It's the "low hanging fruit" of disciplinary actions the supervisors can take. They can nail operators they don't like and make it look like they're doing something important.

The operators are at fault here too. About 90% of them have very poor train handling skills. They have various acceleration and deceleration rates available to them, but they refuse to use them. Instead nearly all of them exclusively use "full power" or "max brake" and nothing in between. That's why you have a such a jerky ride and premature stops.