A SAN is dynamic, and which LUNs are available to a certain host can change based on a number of factors.

The VMkernel discovers LUNs when it boots, and those LUNs are then visible in the vSphere Client. If changes are made to the LUNs, you must rescan to see those changes.

New LUNs created on the iSCSI storage

Changes to LUN access control

Changes in connectivity