Here is a piece of 5/8" solid fiberglass I laid up to cut thru-hull backing plates from. It is bone dry and has never left my shop or been wet other than atmospheric moisture. As you can see it reads a little over 8%.
Different materials can have different readings. This piece of known dry 5/8" solid glass actually reads slightly higher than a dry sandwiched balsa core. Both of these chunks have been sitting in my shop for well over two years, stored right next to each other, so the environmental controls were identical. No need to worry though as any reading on fiberglass between 0% & 15% is considered dry with the CT-33..