The Una and the Lion coin has steadily built a momentum over 15 months in both gold and silver based on a solid foundation of sales across a range of platforms. Many were initially bought by collectors rather than to flip. Substantial price rises weren't widely predicted.
Due to the Una price rises, a lot of people bought the Three Graces to make money (I tried to) due to an expectation that it will do as well despite a higher mintage.
This might be right or it could just be a bubble like many RM coin releases. Time will tell.