The difference almost certainly lies in Civil Defence, aka "Air Raid Precautions". Germany and the UK were well-prepared for air raids, with warning systems, large numbers of air-raid shelters, organised fire and rescue services, and so on.
For your example raids:
- Singapore (1941) had 61 dead, and more than 700 wounded according to the English-language Wikipedia.
- Rotterdam had relatively few deaths because there was enough warning for civilians to flee, but the fires burned out a square mile of the city centre, destroying almost 25,000 homes.
- Belgrade has contested casualty figures between 1,500 and 17,000 deaths, with three to four thousand considered most likely. This is probably due to a lack of civil defense organisation.
- Cologne was bombed many times, but the low number of deaths in the first thousand-bomber raid is due to an effective civil defence organisation. By the end of May 1942, the German defences were running efficiently.
- Dresden suffered a firestorm in February 1945. This overwhelmed civil defence and made air raid shelters useless, because the firestorm consumed all the oxygen from the air, suffocating people in shelters. However, there weren't 250,000 dead, that's a figure from German propaganda. The city authorities estimated 25,000 at the time, and subsequent research has substantiated that estimate.
- Singapore (1944-45) had a total of 11 raids, all of them directed at the naval base, ships, or minelaying. Civilian casualties were avoided as far as possible, and since the defences were poor and most of the raids in daylight, this was mostly successful.
Overall, there doesn't seem to be a case for Axis bombings being more deadly than Allied ones. The number of people killed depends in complicated ways on the amount and type of bombs, the intentions and execution of the bombing, the defences, and the weather conditions, which affect the probability of starting large fires.
The most destructive air raid of WWII was the US Army Air Force's Operation Meetinghouse, where 279 bombers attacked Tokyo with incendiary bombs, starting a firestorm that killed 90-100,000 people and made about a million homeless. This was due to weather conditions that made it easy to start a firestorm, plus the wooden construction of most of Tokyo at the time. Note that this did far more damage than the atomic bombings of Japan.