Why is there a discrepancy between the actual and theoretical molecular weights?

1.Post-translational modifications such as phosphorylation and glycosylation can increase molecular weight.
2.Post-translational cleavage by the protein itself can decrease molecular weight.
3.Some proteins have several splice variants, each with a different molecular weight; these should be distinguished.
4.Related to the amino acid composition (charged vs. non-charged).
5.Formation of multimers can increase molecular weight.