YeaPolska, the usual aspects of condition for stamps also apply to the 613. However, only 49 #613's are known to exist. Things get a bit different for something that rare and valuable. Because of that, the final auction prices vary due to many things other than just condition. The demand for them is extremely high, the supply is extremely low, and very few people who want one can afford it.
The number of people that can afford it and who want one changes from time to time. The number of stamps available to buy at any one point in time varies. If only one rare stamp had been available to buy for period of 20 years, then it might bring a higher price. If all 49 of the 613's had been available for sale every year, then the price would probably lower.
-Edit There's more discussion about this over here if you haven't seen it yet:
https://goscf.com/t/40205