Why is the sensitivity of Mammography low in young females?

Currently mammography is the principal screening modality for breast cancer. It has low sensitivity in breasts with considerable fibroglandular tissue paicularly in young women and in those on hormone replacement therapy. Digital mammography is useful in imaging dense breast tissue. Mammography has a sensitivity of 90% and specificity of 94%. The commonest mammographic abnormality of DCIS is microcalcification other features include asymmetric density and mass. High grade DCIS is associated with linear and branching calcification whereas low grade DCIS is associated with fine and granular calcification. Invasive cancer appear as a speculated mass. Ref: Oxford Desk Reference Oncology By Thankamma V. Ajithkumar, Page 152