Correction - it's nothing osbcure at all, but apparently a matter of the shift that accord broadly with the L sound in Polish a few centuries ago (whereby it became "dark" and velarized), affecting a great many other words and names (like słowo, mały, etc). While in parts east and south the "clear" L sound was preserved.

https://en.wikipedia.org/wiki/Ł

Velarized L is a common phoneme in Slavic languages, inherited from their common ancestor. What makes Polish somewhat unusual is that the pronunciation of velarized L eventually shifted to /w/ pretty much everywhere (a similar process happened in Ukrainian and Belarusian, but only in some contexts).