That's date of first registration in the U.K, confirming it (most likely) wasn't originally supplied new in the U.K
However DVLA records have been known to be wrong.

So it would appear to have been supplied new to some other Country.
It may have been in Jersey, Guensey, Isle on Man, Irish Republic or even Spain etc with an English owner who travelled back regularly.
It also could be someone who worked abroad and knew they were coming back to the U.K but then later returned abroad.

Seems like it may have some non standard history.

DVLA will know where it came from and went to, may be worth trying them to see if they'll tell you that and if it was in the same ownership.