Rivens have dispositions, 1-5. The higher the disposition, the higher the stats will be for example a riven with 5 disposition can get 300+ damage 100+ crit chance, etc. The disposition is determined by how popular a weapon is, DE updates the disposition (maybe 1-2 times a year?) based off data collected from users. If a weapon becomes more popular the disposition will be a lower number, hence the stats on your riven will go down. If a weapon becomes less popular, the disposition will go up increasing the stat numbers. New prime release weapons like Tekko are currently popular because of the new release, so riven prices are up. However, 2-3 months from now the price will dramatically decrease as well as the disposition, which is why DE updates riven dispositions in the first place, to account for this activity. Its done so that riven prices can be priced more accurately is the bottom line.