Leucism is when the cells which produce melanin don't reach theyr destination in the skin. Therefore there are white an if it is incomplete leucism dark aeras in the skin, but both colors - Phäomelanin (red or brown) an Eumelanin (black) will be produced normally wherever the cells are.
Albinism is, when the produktion of the color doesn't work correctly. Therefore only one type of color - pheomelamin or eumelanin - may be affected or both.
In white tigers no part of the striping pattern is lacking, therefore the cells - the melanocytes have reached theyr destination in the skin. But only one color type is affected - the tan peomelanin is lacking. This does only work in albinism but not in leucism.