There are 3 reasons I can think of:
1. When it is flight ready - this increase is due to the idea that concept sales are a good deal, because when they are flight ready it will cost more. CIG needs to sell concept sales to keep their revenue up.
2. Under valuated ships - a ships value is typically tied to a number of factors like it's size, if it's alien, if it's popular with the public. Sometimes CIG don't factor all of this correctly so they under price it. Example is the Starfarer, at the concept time it cost about the same as a super hornet, but it was huge compared to the super, like 10 times bigger lol. Or the Carrack, CIG didn't know it would be so popular when they first concept released it, later price sky rocketed.
3. Inflation - ships came out since 2012. That's 9 years almost. Inflation typically doubles every 2 decades, so CIG have to keep up with inflation as well.