Why is there more variation in proteins than genes?
There are several concepts to understand:


Alternative splicing


From one gene, several proteins can be constructed depending on which introns are spliced.

Tissue-specific genetic expression


Genes are differentially expressed (and differentially spliced) depending on the tissue.

Post-transcriptional modifications


Those are modifications that are made directly on the proteins (in opposition with alternative splicing which are modifications made on the RNA)

Structure of the proteins


Proteins may fold (to what we call primary, secondary, tertiary and quaternary structures) differently in different environment.



Therefore there is variance in proteome between cells although there is no variance in genome between cells (except mutations and red blood cells). Moreover, one gene sequence can give rise to many different proteins due to the processes of alternative splicing and PMT.