Is it normal for the syrup level in a bottle to drop after it cools?

Details:
Glass bottles, 12 oz, 16 oz and 32 oz glass bottles.
Plastic caps both flip top and non flip.
Bottled at 185 degrees and laid on its side for about 1 minute.
Syrup density is 66.5 brix.

So I filtered approx. 7 gallons into my water jacketed canner. Did a final density test at 185 degrees with my refractometer and it read 66.5 brix. Bottled mostly 32 ounce and a few 12 and 16 ounce glass bottles. All the bottles were filled to a 1/4" below the threads for the caps. The bottles were then laid on their sides for approx. 30 seconds to 1 minute. I placed the filled bottles on a stainless cart with a couple inches between every bottle. This process is our usual process and it has always worked for us.

The next morning I noticed that all of the syrup in the bottles had dropped about 1 inch in the 32 ounce bottles and about 1/2 inch in the 12 and 16 ounce bottles. I opened one of the bottles and the seal did do its job and I heard an audible pop when I opened the seal.

Is this normal?? I guess I never noticed it in the past but that doesn't mean it never happened. If that is normal, won't the density change? It can only be evaporation which I'm not sure how that is possible in a sealed bottled. Am I missing something?