Blood content doesn't tell us tissue content. We use blood content as markers for tissue content, but often it's misleading because we do straight correlation, i.e. if there's more in the blood there must be more in tissues. There's more possible reasons for blood content.

A priori, it's all about a balance between growth and recycling. There's continuous growth, and continuous inhibition of growth. If anything interferes with growth or inhibition, balance will shift, it will show up in the blood. In this sense, a higher calcium count indicates there's not enough growth or too much inhibition. Tissues can't keep what they got, it gets dumped in the blood - higher calcium count.

Then, it's a demand system - the blood is the supply pipeline for everything. The greater the demand, the more there's gonna be in the blood. We can see this with ketones, glucose, various hormones like GH and testosterone for example, and a bunch other things. In this sense, higher calcium indicates greater demand.

Considering the many beneficial effects of low-carb, it seems unlikely to me that a lower calcium count is anything to worry about. If anything, it shows that things are returning to normal - growth/inhibition balance, proper demand response.