This has come up a few times - I guess you would describe it as a "feature" of the way Clarity handles the finish date!
See ; Project Finish Date in Clarity
and Q: Planned Cost Finish - different valude displayed in Clarity / Database ?
COP_CALC_FINISH_FCT Function
for some discussion around this.
(bottom line - Clarity is "right", reading the data directly off the database is "wrong")
COP_CALC_FINISH_FCT is the database function that "converts" the database-date to the Clarity-date