I'd take the bulbs out and see if they're the same part number, and the same manufacturer. Some bulbs are available in higher output versions, and aftermarket coated bulbs can have higher wattages than OEM bulbs.
Could one headlamp be aimed higher than the other? Did they mention the word "aim" at all?
Is one light newer than the other? Halogen lights will dim as they age, and should be replaced in pairs.
If that is not the case, then you could have a loose ground somewhere. Do you notice any change in brightness of the dim bulb when DRLs are turned on/off? Check for signs of corrosion when you take off the wiring connector.
Make sure all three wires are connected, and that the contacts on the bulb socket are clean.
The WHITE with LIGHT GREEN STRIPE wire is the common wire on the bulb socket that leads to a separate fuse in the Battery Junction Box under the hood. If that wire isn't connected on one side (or if the bulb isn't making good contact to it), the low-beam filament will form a series connection with the high-beam filament on the other side, and it will appear dimmer.