I believe it's because Belgium, having been ranked above 30th in the Nations cup last year, were eligible to start 2 and register 4 athletes. The World Champs level for qualifying points must be much lower (we saw Turkey and Greece, for example) so they managed to find 4 athletes who qualified, probably by virtue of last year's results or this year's IBU cups. The wildcard allocation for the championships really confused me though. China did pretty well in the early IBU Cups, yet they get no spots. Turkey, however, gets two.