Demand for foreign exchange is continuous throughout the year but the supplies come in at discrete intervals. This leads to periodic tightness in the domestic foreign exchange market which the Central Bank alleviates by selling foreign exchange from its stock of official reserves to the banks.
Second, until April 2014, the Central Bank was using a twenty year-old system to sell and distribute foreign exchange to the banks. As an aside, I was part of the team at the Central Bank that instituted this distribution system following liberalization of the foreign exchange market in April 1993. This system served the market well, particularly in the early years.