Hello and welcome to the forum. I know it is always scary when you get the call back for more investigation. This does not mean they found anything, however, just that they want to take a closer look. Density occurs often as a woman gets older. They tend to look more closely when that is the case. Here is some general information on breast density. https://www.webmd.com/breast-cancer/dense-breasts#1 Just having breast density can trigger a radiologist to want to take a closer look to make sure they aren't missing anything. Fibroglandular densities are a common finding on mammograms. 40% of women have this. This is not a cancer finding but it just again makes it harder for the radiologist to see and examine breast tissue.
While your report isn't 'suspicious' it is unclear and that is why they are taking a second look. Try to remain calm as most of the time, the second look gives you an all clear. Please let us know how it goes!
The purpose of the initial screening mammogram is just to find anything that catches the radiologist's eyes. These might include focal asymmetries and/or clusters of microcalcifications, the former which would be worked up with additional spot compression views and the latter which would be worked up with spot magnification views. At this point, there is no reason to get overly concerned. Just make sure you return to get the recommended diagnostic mammogram. Most of the time those focal asymmetries press out with spot compression views, suggesting normal superimposed fibroglandular tissue. If the abnormality persists with spot compression, then the next step would be biopsy. Good luck!
On an unrelated note, I also wanted to explain the 4 breast composition categories:
a. The breasts are almost entirely fatty.
b. There are scattered areas of fibroglandular density.
c. The breasts are heterogeneously dense, which may obscure small masses.
d. The breasts are extremely dense, which lowers the sensitivity of mammography.
The first 2 are considered not dense; the latter 2 are considered dense.
In your case, you fell into category b, which is technically not dense.
The more dense a breast is, the harder it is to detect underlying masses.
This breast composition category has nothing to do with why you were called back.
This breast composition category is included with every mammogram report.
It is used more as a qualifier to explain the study's limitations.