Not sure what BRS system you have, or how the TDS meter is hooked up, but *most* of the dual TDS meter systems measure the water coming in to the DI, and going out of the DI. That way, you know how much your RO membrane is taking out and when your DI is going bad.
Most likely, the first number is the TDS of the water coming out of the RO membrane. It's *not* the TDS of your tap water. The second number is your output, or the TDS coming out of the DI.
Again... just guessing on this, but this is normally how dual TDS meters are hooked up.