Are you measuring solutions in two separate beakers and pouring them together into a third or are you measuring 100 ml. of the first solution into a beaker then filling with the second solution up to 200 ml.?
Try this experiment: Get some 90% isopropyl alcohol (from the drug store) and measure 50 ml. into a beaker or graduate. Next, using a second container, measure out 50 ml. of plain water. Pour them together. Note the volume of the combined solutions.
You would expect the volume to be 100 ml. but it's not. It will be less.
The reason is because the molecules of water and alcohol are different sized. The molecules of alcohol slip into the spaces between the water molecules. It's similar to what would happen if you poured a bucket full of sand into a bucket full of marbles. The total volume will be less because the sand is going between the marbles.
The answer would be to measure two solutions separately and pour them together into a third container.