As far as Islamic Sharia is concerned, zakah is one of the main pillars of Islam and it is obligatory like the five daily Prayers. Zakah is obligatory on those who have the nisab (Arabic word for: the minimum  amount of wealth one must have before he is liable to pay zakah).
A Muslim is obliged to pay zakah on his or her wealth if the conditions of paying zakah (namely, having the nisab and the passage of one full lunar year while having the nisab) applies to him or her. The completion of the lunar year may differ from one person to another. But many Muslims are keen to give out their zakah during Ramadan as we are told that rewards for good deeds (even if obligatory) are multiplied in this blessed month.

In his response to this issue, Dr. MonzerKahf, a prominent economist and counselor, stated,

Zakah for every person is due on lunar year after he or she owns the amount of nisab for the first time, provided that he or she owns also at least the same amount at the end, that is 354 days and then after every lunar year.

This means that every Muslim would have his or her own fiscal year for zakah, and only a few of them may happen to have that due date coincide with Ramadan.
This is the principle, but since we know that the reward from Almighty Allah forgood deeds is higher in Ramadan than in other days of the year (as told by Prophet Muhammad, peace and blessings be upon him) many Muslims like to adjust their “zakah year” to come in Ramadan so that they get more rewards from Allah. This adjustment is done by paying an extra amount in proportion of the difference in the number of days until Ramadan.