There's quite a bit of variability, depending on the reporting. This 3% value, which was issued in January of this year from DFO science, is the lowest that DFO has used in the past 20 years. They also use a lower mean body weight for adult harp seals. It's been reduced from a previous 140 kilograms per animal down to 110 kilograms, so 3% of 110 kilograms is where you get the 3.3 kilograms.
In terms of other science and the report in particular that references 7 kilograms to 9 kilograms, I remember that one quite well, because it was quite easy to read. The researchers indicated that 9 kilograms per day was required for female seals—because they carry young, they need more food—and 7 kilograms for males.
That particular study also went on to include the types of prey fish that were being eaten. Fatty fish like herring, mackerel and capelin would generate 2,000 calories per kilogram, and lean fish and whitefish would generate 1,000 calories per kilogram. Depending on the type of fish and the availability, it influences the amount, but if you study the numerous reports that are out there on seal diet right across the international community, you will see variability.
To give an example of stationary seals in an aquarium, SeaWorld reports that they need 5%. If they're in the wild, you can assume that they would need more food than seals in an aquarium.