Abuse of card payment by Uber.

When you request a ride you get options with the amount to pay. You make your choice and the amount is deducted from your bank account as you commence the trip. As the trip ends another amount gets deducted from the same trip. The second deduction cost is more than the initial amount. Later, the smaller amount is reversed to the bank. How do you get charged two times for a single trip and different amounts. You are misled with the cost of the trip. Why abuse the access to the bank card. I have had this experience with Uber more than once, meaning that it's how they conduct the business now. Why?