Wouldn't surprise me.
The Mid-Atlantic Ridge is constantly pushing up new material from the Earth's mantle, some (~6% by weight) of which may have magnetic properties (iron). An extra-large "burp" of such might result in a local magnetic anomaly under the ocean.
The outflow also "records" reversals in the Earth's magnetic field over the millennia, also forming anomalies:
http://earthref.org/drupal/sites/ear...anomalies2.jpg
(Big Image):
http://www.geology.sdsu.edu/kmlgeolo..._anomalies.jpg
And the charted declination is generally based on a model, rather than direct point-by-point measurements (which would be hard to do consistently every 90 days in mid-ocean anyway).
Plus, our old friend the solar wind pushes on the magnetosphere, and can cause brief distortions (cf: geomagnetic storm).