In the United States - generally speaking - the usual check-in time is 3 O'clock (15:00), with check-out being Noon (12:00). For some hotels, the check-out might be as early as 11 O'clock (11:00).
Now - does this mean that there are no rooms available until 15:00? No. I've checked-in as early as 09:00...but only because there were rooms which were available for assignment by that time.
If you are checking into a hotel which has just hosted a large convention, or in a town which has just hosted a major sports competition or championship or festival, there will be a lot of rooms which will likely be vacated by their previous occupants rather late, and this will give the Housekeeping Staff minimal time in which to service the rooms - changing the sheets and making up the beds, cleaning the lavatory/toilette and shower, vacuuming the floor, stocking the mini-bar and the stationary/amenities for the writing desk - all to make ready for you...and doing it for most (even perhaps all) of the rooms of the hotel, within that 3-hour window for meeting that deadline of 15:00.
What to do? If you drive into Berlin, leave your baggage in the boot of your car, gently inquire at the registration desk, and be prepared to either visit the bar, or walk about the premises, getting the "lay of the land" and a feel for our temporary home.
If you are flying in or taking the train, pick a flight or train which has a mid-day arrival time.