The skimmer might remove a little calcium as it will fairly form ionic bonds with some of the waster materials . Along with the fact that it will get caught up with some of your other skimmate but, it would only be a very small amount not enough to affect readings. However, if you are having ph fluctuations it will start to come out of solution.
hth
Max
Oh, I also hope that you are testing for any additives that you add to your tank,"exception being a couple of drops of iodine a month." In most cases you'll be a lot better off having a deficiency vs an overabundance.
Sorry, if you already knew that saying it more for anyone who doesn't.