We booked a room in advance one month before the stay. We confirmed our arrival the previous week and the day before, telling the hotel staff how we would arrive to Siam Reap and aprox at what time. When we were already there, at the bus station, there was nobody waiting for us, as we had arranged. They had decided to give our room to someone else just because they didn't know with which bus company we were coming. They thought we had been conviced by the bus company to stay at their guesthouse.
I found it really disappointing. When you are traveling sometimes you are not carrying a mobile phone with you all the time and perhaps you cannot so easily be contacted, and you might not have acces to the internet all the time. I think they didnt act seriously. It is not a good policy for a mid range-expensive Hotel to leave someone on the street with a one month reservation, just because they dont know on what bus station you are stopping. Specially at 2 pm, I dont think it was too late for arrival
Anyway. I hope this is usefull for other travellers. I understand that it is a risk for them to accept bookings via the internet, but if they do, they have to assume this risk!!!