Here's why:

* Brix is a measurement of the sugar content in a solution, typically expressed as percentage by weight. It is determined using a refractometer, which measures the refractive index of the solution.

* Total soluble solids (TSS) refer to all the dissolved substances in a solution, including sugars, salts, acids, and other soluble compounds.

* In many cases, sugars are the dominant component of TSS, particularly in fruit juices and syrups. Therefore, Brix provides a good approximation of the total soluble solids content.

However, it's important to note that Brix is not a perfect measure of TSS:

* Brix only measures sugars, while TSS includes all dissolved compounds.

* The relationship between Brix and TSS can vary slightly depending on the composition of the solution. For instance, solutions with high levels of non-sugar solids may have a slightly higher TSS compared to their Brix reading.

In summary:

* Brix is a quick and convenient way to estimate the total soluble solids content of a solution.

* While not a perfect measure of TSS, Brix is a good indicator of the overall concentration of dissolved compounds in many liquids, especially those rich in sugars.