to quote "SpaceFlight Now" space news (June 14th 2003) "Flattest Star Ever Seen":

The high degree of flattening measured for Achernar - a first in observational astrophysics - now poses an unprecedented challenge for theoretical astrophysics. The effect cannot be reproduced by common models of stellar interiors unless certain phenomena are incorporated, e.g. meridional circulation on the surface ("north-south streams") and non-uniform rotation at different depths inside the star.

http://spaceflightnow.com/news/n0306/14flattest/

And to quote: ScienceNow (12 June 2003)

Achernar turned out to be much flatter than expected, Kervella says. In fact, its polar diameter is a mere 64% of its equatorial diameter (the previous record was 85%). That was a surprise, given that the spin rate of 225 kilometers per second suggested a flattening of only 80%. One possible reason is that gas inside a star may spin even faster than the gas on the surface, which would bloat its midriff, says Grant Bazán of Lawrence Livermore National Laboratory (LLNL) in California. Studying that effect would require a three-dimensional model of the star, says Kervella.

http://www.govertschilling.nl/artikelen/science/030612_sc.htm