"Lock" does seem somewhat of a misnomer if the FAFC/ECC is still free to chase the target.
Lock applies to certain inputs (TMC trim, bleed debits, OAT), but not to throttle position or closing on EPR (or N1) Command.
TAT is also related to speed and altitude.
The difference is the real world temperature doesn't react consistently. Although the relationship between OAT and TAT with increasing speed is well defined, the OAT may change, and while there is an ideal relationship of temp with altitude, the real world doesn't necessarily match that (e.g. temperature inversions). In a highly simplified sense, lock and lapse assumes a consistent OAT, regardless of what the real OAT does.