Finasteride blocks two versions of the enzyme alpha5-reductase. This enzyme converts testosterone to dihydrotestosterone. The drug does not greatly affect other systems outside of those needing this particular enzyme.
Spironolactone on the other hand does directly interface with cell receptors. The androgen receptor is a secondary point for it. It primarily latches to mineralcorticoid and glucocorticoid receptors. It's the activity in those systems that causes increased water egress but does not trigger an increase in potassium flushing. This is why potassium levels can become dangerously high if sodium gets flushed out too much.