because one large donation incurred fewer fees (including payment processing fees) than several smaller donations
Nonprofits Want Monthly Donations Too
Nicole Dieker

The difference in fees between 12 monthly donations and one annual donation would be $3.60 for most payment processors, plus whatever staff time is required to do the extra bookkeeping. For small nonprofits without cash reserves, the cash flow benefits of monthly donations far outweigh this negligible extra cost.