If you are shooting in 16:9 aspect ratio then 16 MP is the greatest you will see. If you shoot 4:3 ratio you should see 18 (maybe 19, I'm going off my 1520).
Other camera Apps like ProShot or Camera360 Sight may allow more within those ratios.
It has to do with what is trimmed off the 1:1 sensor to create an image. This is normal on all 20 MP models: 1520, 930/Icon, 950 and 950 XL.