It's the gearing...used for spool oscillation. Happens on all reels all across the board. Not a single manufacture will escape that...
Well one can hide it on a short spool stroke. You won't see it.
Shimano tends to over extend their spools on the full stroke of the oscillation to accommodate larger line diameter. When you use smaller line diameter and the worse case scenario happens...you get an ugly line wrap on the spool..
Seen it on my Shakespeare, Pfleuger, Quantums, Abu's, Diawa's, Mitchell, and yeah, my Shimano's too. Does it happen everytime, no. When it happens, it happens...It's like that stupid pot hole that happens on the road....when it happens it happens...
When you get to the Shimano Stradic, it used a worm gear for the spool oscillation and it does a better job at a more even line lay...Ugly line lay on the spool may still happen. If all the conditions for that to happen, will happen.