NADA GUIDES shows increase in 355 prices

I recently bought a 355 f-car. During my 9 month long search I used NADA GUIDES as a benchmark for the prices of 98/99 355 spiders. I found it to be very helpful and accurate. People are asking all kinds of #'s for these cars, but in the end when they finally sell them the price closely reflected given range. If a car was to be traded into the dealer, the price they would pay was very close to "low retail", and a clean top of the line car would sell for "high retail" figures. I saw this time after time with some of the deals I came close to making - the GUIDE was right on!
SO, imagine my surprise when a week after I bought my car, and went onto NADA site to see how much $ I lost, I saw that the car actually went up in rice 10k! I did not see this rebound last spring. As the matter of fact the prices dropped dramatically then. I don't think it's seasonal. The car shows now to be the same price it was a year ago. It dropped in December the same 10k, and now it is back where it was.
What's up with that?
Did the bleeding on 355's finally slow down to a trickle?
Did these cars find a plateau until the new model is introduced?
What do you think?

 

Just in case here is the link
http://www.nadaguides.com/Values/ValueCategoryYear.asp?UserID=530726C4BB1F1&DID=37844&wSec=1&wPg=1204&CategoryId=4&MakeId=1268&ColorId=

 

I am saying that the NADA is just a guide, and a pretty accurate one at that. In December 98 6speed spi high retail was $95,100. I am sure they do not take into consideration 30k service, clutch replacement, challenge grill and so on which can easily add 10k to the price. Also understand that eventhough for the purest Ferrari enthusiast 6 speed is desirable, the market discounts for it vs. F1. (which was $102,800 in December)