I would think that the sample had sulfate from another source(an impurity) OR that the alum, when it was prepared, had an excess of one of the reagents. Alums are double sulfate salts; i.e. K2SO4 + Al2(SO4)3 with water of hydration. Typical formula is KAl(SO4)2.12H2O. When prepared from the ingredients either an excess of K2SO4 or Al2(SO4)3 could be present which would increase the sulfate. The third possibility is that the analytical procedure for the determination of sulfate (I assume BaSO4 was the precipitant) had a problem so too much BaSO4 (or what looked like too much BaSO4) was obtained. (What looked like too much BaSO4 could be BaSO4 not dried completely.)
