Google introduces a automatically calculated monthly charging limit for unchanged budgets, based on your daily budget. "When your daily budget is unchanged throughout a month, We'll set a monthly charging limit for that campaign by multiplying your daily budget by 30.4, which's roughly the average number of days in a month (365 days in a year / 12 months = 30.417 days/month). For e.g., if your daily budget is US$10 throughout a month, you won't be charged more than $304 during that month (US$10 daily budget * 30.4 average days per month). As usual, actual daily spend may vary," explains Google.
Accounts will transition to monthly charging limits over next several weeks. When you edit your budget from Campaign Settings, a new tooltip will also be visible describing this change.