Use Digital Multimeter to measure Copper thickness… it is not convincible and has inaccuracy.
It have to be measured by professional machine to see the copper thickness, like the attached picture shows. 
For 1.6mm 2 layer board, the orginal material is already 1.5mm , and has 0.5oz each side. Then we will go through all the process including laying copper. The outcome copper of the board will be really near 1oz each side.
You can use machine to measure the copper thickness. Or ship one of your board to us. We can test it and show you the specific copper thickness. Thanks.