Why do α-hydroxy ketones give Tollens' test?
The Tollens' test is done in base, which mediates tautomerization through the enolate:



The misconception that you have is that all alpha-hydroxy ketones can give a positive Tollens' test. Aldehydes give a positive test, and so only alpha-hydroxy ketones that can tautomerize to aldehydes will give a positive Tollen's test. My example above is 1-hydroxy-2-propanone, which isomerizes to 2-hydroxypropanol. Most alpha-hydroxy ketones with names that match the pattern 1-hydroxy-2-alkanone will give a positive Tollens' test.

Benzoin will not give a positive test, since it cannot isomerize to an aldehyde. What does benzoin form upon tautomerization?