How do you interpret and manage discordant HER2 in IDC breast cancer between primary tumor and lymph nodes?

Trastuzumab and Pertuzumab have made such significant contributions to decreasing breast cancer mortality that I would give irrespective of which site (breast or axillary) is positive. Speculate as why the breast primary is HER2 overexpressing and the nodes are not are 1) falsely negative assay for ...