To crawl or not to crawl
The question here is: is it proper to crawl at a, or would that be the route to defeat?
This time, Black's move is not sente against the life of the corner.
Therefore, White's crawling once more does nothing more than crawling, which is the route to defeat. Black happily expands his influence from the third line.
Note: There is a reason that White plays before in the joseki. After the exchange, - , Black is already strong, and is less likely to reply to with .