Google can bill you two separate times – first, if you meet your payment threshold before the bill date, and once more on your bill date. If you meet the payment threshold amount before your bill date, Google will immediately charge your card in that amount. You will then be charged any remaining amount of ad spend on your bill date.
Google also has the following articles that are very helpful and goes into further detail about the payment threshold on your account, why you’ve been charged, and how charges appear on your statement.
Payment threshold: https://support.google.com/google-ads/answer/6311
Understanding why you’ve been charged: https://support.google.com/google-ads/answer/2375373?hl=en&ref_topic=3123053
How Google Charges appear on your statement (the middle section of the article): https://support.google.com/google-ads/answer/2472641?hl=en