1780 2 Reale Purity Question

Possibly because one was issued in Spain and the other in Cuba.
Also note the Cuban coin is slightly thicker and weighs more.

The "Cuban" pieces are simply peninsular Spain (aka the actual country of Spain proper) coins that were countermarked. The mark didn't indicate any kind of metallic content difference.

The Krause catalog has caused endless confusion on the specifications of the milled shield and portrait Spain 2 Reales. The Spain proper issues were in fact around 6.0g weight... and yes, with that lower .81 purity. The net result of that is that it took FIVE of these peninsular 2R coins to equal one Spanish peso, or 8 Reales, coin - as opposed to FOUR, which was still the case for the Spanish COLONIAL 2R. This was an intentional devaluation by the Spanish crown that was fully in effect by the early 1700s.