Yes, I have found out about Curly Tops as well, though these seem to be much more... curly
Anyway, the comment that JRsbugs made prompted me to check them again.
The colors differ slightly. The older ones are more bluish to darker green further towards the center while all the newer ones are pale blue to light green. Then there are the needles, somewhat wavy on the older ones, shorter and straight on the newer ones.
Could it be that they are actually two different cultivars (the comparison images will have to be clicked for full size)?
(left: newer plants, right: older plants)
