You cannot assume that the adcom accepted significantly more students than in previous years based on a a dearth of offer-withdrawals. Statistically speaking, more acceptances would result in more withdrawals (albeit still with few coming off the WL).
A lack of withdrawals more likely signifies one (or more) of several plausible conditions:
a. The adcom selected competitive students which they felt were genuinely interested in Keck and would most likely attend, even if accepted to other institutions.
b. The adcom selected students who were less competitive than in previous years, meaning Keck would be their only / top choice.
c. The adcom & faculty did a particularly outstanding job convincing interviewees & accepted students of the merits of Keck. Consequently, no one wishes to give up their seat.
d. With the consecutive year-on-year increases in tuition / student fees at UC schools, the financial incentive for attending a public CA school over USC has decreased significantly.
e. With the construction of the new LAC hospital, Center for Regenerative Medicine, Norris Cancer Center, and planned procurement of $1.5 billion in funding, Keck may simply be a more promising / attractive institution than ever before.
It may be a combination of all these factors. In any case, I wish you the best of luck in the painful and de-humanizing process that is medical school admissions.