We don't have the right figures for all species. There's a new way of calculating the number of grey seals, and the people responsible for that are preparing a new way to calculate the hooded seal population. The seal population of Greenland is estimated every five years, but that's not done for the other species because they aren't hunted as much.
The true seal population isn't concentrated like the grey seal, hooded seal and harp seal populations. It's a lot easier to estimate the harp seal population because they form large concentrations every year for newborns. In addition, scientists include baby seals, if you will, in their population estimates. The other seal species aren't found in such large concentrations.