Alphabet's buyback program distorts the price differential between the two types of shares. In April, Alphabet announced a new $50 billion buyback program for its C shares, i.e. GOOG, whereas there will be no buybacks for GOOGL. This buyback activity has distorted the supply-demand picture, at least to some degree, and explains why GOOG shares are currently slightly more expensive than GOOGL shares. It seems very much possible that the current premium of GOOG over GOOGL fades away over time, as the initial buyback frenzy cools down.
https://seekingalpha.com/article/4430928-which-google-stock-is-a-better-buy-goog-or-googl