How can lawyer charge for 1 hour of time when only 15 was used

The attorney is required to follow the terms of the retainer agreement that the two of you should have signed when you first hired him. It is common to calculate time in 15 minute incrementa, meaning that if less than 15 minutes it is billed as 15 minutes, if 16 minutes it is rounded up to 30 minutes and so on. One hour minimums are very rare, I have never seen any firm that does that. You may have a valid fee dispute if there is no contract or if the contract was not followed.