I don't think you are getting it. Both FW and LW connect to exactly the same network, so essentially FW can't "have" more music than LW. FW can however be better coded to connect to better Ultrapeers. OR more likely (since both are based on the same code) it's a luck of the draw.

Try searching in both after being long connected (several hours) as UP instead of a leaf to prevent misleading results.

Another scenario could be that more Gnutella users using Frostwire as a client are sharing your type of music. And it's known that client programmers often make their client prefer their own kind as they say.

This does not change the fact that all the music is on the Gnutella network being shared by users using all sorts of clients, so maybe a non-discriminatory client would find much better results than FW or LW.