Can a browse node have more than one search index?
Yes, a browse node will often return results for more than one search index.
Sometimes those results make sense - for example when a node has products in both the Apparel and Merchants indexes - but, most of the time, the other indexes either return the same products as the "most appropriate" index for the node or they return products that have no relation to the node.
For example, the MP3 downloads nodes return the same results for most every index.
Another example is the main Apparel node (1036592) which returns appropriate, but different, products in the Apparel, Jewelry, Merchants, and Shoes indexes. The Apparel node, however, returns inappropriate products for every other search index.
See Also: A Simple Way to Determine the Search Index for a Browse Node