To answer your question: Are there other explanations to a vibration in the steering wheel that changes proportionally with speed (and only occurs when braking)?
Modern Rotors warping after installation is a myth. Cheap rotors come eccentric, or with uneven thickness, and/or they get installed crooked. Then pad material builds up on the high spots, create more friction than the rest of the rotor, and you get the pulsation feeling.
Every factory service manual has specs for rotor runout and parallelism. Every dealer and shop ignores them, slaps the rotors on and sends it out the door. Why? They can make more money faster doing more jobs, and usually can blame the owner for "warped rotors" when they come back.
A second possibility is improper bedding, where a pad imprint causes friction material buildup on one part of the rotor more than the rest. The pad imprint comes from standing on the brakes while stopped, with overheated rotors. This doesn't happen to commuters much, and is more of a performance driving issue.
