Its just a preference of people to write it with two 2 Ts or 1. Hindu members of the community usually spell it with one T while Sikh and Muslims will put 2 Ts. Mostly because most Hindu Jats speak languages other than Punjabi as primary language so in their language Jat is correct way to romanize the word. Old history books and encyclopedias though mostly have it with one T and it designates the whole community.

Usually some people like to argue that Hindu Jats and Sikh and Muslim Jatts are different communities but anthropological evidence shows they are the same and follow the same customs for birth, marriage, death usually.