I did an experiment once on a guitar that had both an L90 and L500 installed. I played back white noise through a pair of earphones which I placed over the pickups while monitoring the output. One thing I noted above all was that the L90 was much more sensitive just above the blades picking up virtually nothing between the blades while the L500 picked up an almost equally strong signal between the blades as directly above.
I'm sure this counts for some of the tonal differences and is certainly stated in the official product literature for the L-90 and L-45: "...a tightly focused magnetic window..."