HSG planned surprise w insurance

So I met with a new doctor today to get a second opinion on our TTC trouble and decided to plan for a HSG for January. I didn’t think my insurance would cover it so was doing research on what to expect price wise. So I called my insurance and asked if it would be covered( I even spelled out hysterosalpingogram to them to make sure we were on the same page) and to my surprise she said it would be covered 100% as it’s coded as “preventative” I am lost. Not that I’m complaining that it’s covered I’m just confused as to why it’s coded that way. To me it sounds too good to be true and will afterwards be billed out the wazoo. Any input?